require ruby 2.2+ in gemspec (#1506)

* require ruby 2.2+ in gemspec

As of 0ed4ba8, we are no longer running CI for ruby < 2.2. Per #1440,
puma hasn't actually been compatible with ruby 1.9 since puma 3.8.0.

github: fixes #1440

* Don't run ruby 2.1 in appveyor either

* Remove obsolete 2.1-Gemfile

.gitignore

@@ -12,7 +12,6 @@ tmp
 t/
 .rbx/
 Gemfile.lock
-gemfiles/2.1-Gemfile.lock
 .idea/
 /test/test_puma.state
 /test/test_server.sock

gemfiles/2.1-Gemfile

@@ -1,14 +0,0 @@
-source "https://rubygems.org"
-
-gemspec path: ".."
-
-gem "rdoc"
-gem "rake-compiler"
-
-gem "rack", "~> 1.6"
-gem "minitest", '~> 5.9'
-gem "minitest-retry"
-
-gem "jruby-openssl", :platform => "jruby"
-
-gem "rubocop", "~> 0.50.0"

puma.gemspec

@@ -18,5 +18,5 @@ Gem::Specification.new do |s|
             %w[History.md LICENSE README.md]
   s.homepage = "http://puma.io"
   s.license = "BSD-3-Clause"
-  s.required_ruby_version = Gem::Requirement.new(">= 1.9.3")
+  s.required_ruby_version = Gem::Requirement.new(">= 2.2")
 end