Update Rubinius build setup

TL/DR; use rubinius 2.2.5 in development

Rubinius 2.2.5 now includes the standard library, so we do not need to
explicitly include standard library gems in Gemfiles.

http://www.benjaminfleischer.com/2013/12/05/testing-rubinius-on-travis-c
i/

.travis.yml

@@ -4,7 +4,7 @@ rvm:
   - 2.0.0
   - 2.1
   - jruby
-  - rbx
+  - rbx-2
 gemfile:
   - test/gemfiles/Gemfile.rails-3.2.x
   - test/gemfiles/Gemfile.rails-4.0.x
@@ -12,7 +12,7 @@ gemfile:
 matrix:
   exclude:
     - { rvm: jruby, gemfile: test/gemfiles/Gemfile.rails-3.2.x }
-    - { rvm: rbx, gemfile: test/gemfiles/Gemfile.rails-3.2.x }
+    - { rvm: rbx-2, gemfile: test/gemfiles/Gemfile.rails-3.2.x }
 
     - { rvm: 1.9.2, gemfile: test/gemfiles/Gemfile.rails-4.0.x }
     - { rvm: 1.9.2, gemfile: test/gemfiles/Gemfile.rails-4.1.x }
@@ -24,7 +24,7 @@ matrix:
   allow_failures:
      - { rvm: jruby, gemfile: test/gemfiles/Gemfile.rails-4.0.x }
      - { rvm: jruby, gemfile: test/gemfiles/Gemfile.rails-4.1.x }
-     - { rvm: rbx, gemfile: test/gemfiles/Gemfile.rails-4.0.x }
-     - { rvm: rbx, gemfile: test/gemfiles/Gemfile.rails-4.1.x }
+     - { rvm: rbx-2, gemfile: test/gemfiles/Gemfile.rails-4.0.x }
+     - { rvm: rbx-2, gemfile: test/gemfiles/Gemfile.rails-4.1.x }
   fast_finish: true
 script: "bundle exec rake submodules test"

Gemfile

@@ -13,8 +13,3 @@ end
 platform :mri_21 do
   gem "simplecov"
 end
-
-platform :rbx do
-  gem 'rubysl', '~> 2.0'
-  gem 'racc'
-end

test/gemfiles/Gemfile.rails-3.2.x

@@ -2,8 +2,3 @@ source "https://rubygems.org"
 
 gem 'rails', '~> 3.2.0'
 gemspec :path => "../.."
-
-platform :rbx do
-  gem 'rubysl', '~> 2.0'
-  gem 'racc'
-end

test/gemfiles/Gemfile.rails-4.0.x

@@ -8,9 +8,3 @@ end
 
 gem 'rails', '~> 4.0.0'
 gemspec :path => '../..'
-
-platform :rbx do
-  gem 'rubysl', '~> 2.0'
-  gem 'racc'
-  gem 'json'
-end

test/gemfiles/Gemfile.rails-4.1.x

@@ -2,8 +2,3 @@ source "https://rubygems.org"
 
 gem 'rails', '4.1.0.beta1'
 gemspec :path => '../..'
-
-platform :rbx do
-  gem 'rubysl', '~> 2.0'
-  gem 'racc'
-end