Bump required Ruby version to 2.1.0

[This article](http://weblog.rubyonrails.org/2014/8/20/Rails-4-2-beta1/#maintenance-consequences-and-rails-5-0) states that:

> Rails 5.0 is in most likelihood going to target Ruby 2.2.

Before the exact minimum version is fully decided, @arthurnn [suggests](https://github.com/rails/rails/pull/17830#issuecomment-64940383)
that **at least** version 2.1.0 **must** be required by the `gemspec` files.

actionmailer/actionmailer.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |s|
   s.summary     = 'Email composition, delivery, and receiving framework (part of Rails).'
   s.description = 'Email on Rails. Compose, deliver, receive, and test emails using the familiar controller/view pattern. First-class support for multipart email and attachments.'
 
-  s.required_ruby_version = '>= 1.9.3'
+  s.required_ruby_version = '>= 2.1.0'
 
   s.license = 'MIT'
 

actionpack/actionpack.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |s|
   s.summary     = 'Web-flow and rendering framework putting the VC in MVC (part of Rails).'
   s.description = 'Web apps on Rails. Simple, battle-tested conventions for building and testing MVC web applications. Works with any Rack-compatible server.'
 
-  s.required_ruby_version = '>= 1.9.3'
+  s.required_ruby_version = '>= 2.1.0'
 
   s.license = 'MIT'
 

actionview/actionview.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |s|
   s.summary     = 'Rendering framework putting the V in MVC (part of Rails).'
   s.description = 'Simple, battle-tested conventions and helpers for building web pages.'
 
-  s.required_ruby_version = '>= 1.9.3'
+  s.required_ruby_version = '>= 2.1.0'
 
   s.license     = 'MIT'
 

activejob/activejob.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |s|
   s.summary     = 'Job framework with pluggable queues.'
   s.description = 'Declare job classes that can be run by a variety of queueing backends.'
 
-  s.required_ruby_version = '>= 1.9.3'
+  s.required_ruby_version = '>= 2.1.0'
 
   s.license = 'MIT'
 

activemodel/activemodel.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |s|
   s.summary     = 'A toolkit for building modeling frameworks (part of Rails).'
   s.description = 'A toolkit for building modeling frameworks like Active Record. Rich support for attributes, callbacks, validations, serialization, internationalization, and testing.'
 
-  s.required_ruby_version = '>= 1.9.3'
+  s.required_ruby_version = '>= 2.1.0'
 
   s.license = 'MIT'
 

activerecord/activerecord.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |s|
   s.summary     = 'Object-relational mapper framework (part of Rails).'
   s.description = 'Databases on Rails. Build a persistent domain model by mapping database tables to Ruby classes. Strong conventions for associations, validations, aggregations, migrations, and testing come baked-in.'
 
-  s.required_ruby_version = '>= 1.9.3'
+  s.required_ruby_version = '>= 2.1.0'
 
   s.license = 'MIT'
 

activesupport/activesupport.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |s|
   s.summary     = 'A toolkit of support libraries and Ruby core extensions extracted from the Rails framework.'
   s.description = 'A toolkit of support libraries and Ruby core extensions extracted from the Rails framework. Rich support for multibyte strings, internationalization, time zones, and testing.'
 
-  s.required_ruby_version = '>= 1.9.3'
+  s.required_ruby_version = '>= 2.1.0'
 
   s.license = 'MIT'
 

railties/lib/rails/ruby_version_check.rb

@@ -1,13 +1,13 @@
-if RUBY_VERSION < '1.9.3'
+if RUBY_VERSION < '2.1.0'
   desc = defined?(RUBY_DESCRIPTION) ? RUBY_DESCRIPTION : "ruby #{RUBY_VERSION} (#{RUBY_RELEASE_DATE})"
   abort <<-end_message
 
-    Rails 4 prefers to run on Ruby 2.1 or newer.
+    Rails 5 requires to run on Ruby 2.1 or newer.
 
     You're running
       #{desc}
 
-    Please upgrade to Ruby 1.9.3 or newer to continue.
+    Please upgrade to Ruby 2.1.0 or newer to continue.
 
   end_message
 end

railties/railties.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |s|
   s.summary     = 'Tools for creating, working with, and running Rails applications.'
   s.description = 'Rails internals: application bootup, plugins, generators, and rake tasks.'
 
-  s.required_ruby_version = '>= 1.9.3'
+  s.required_ruby_version = '>= 2.1.0'
 
   s.license = 'MIT'