Update Rubinius Gemfiles, Travis

Simplify the Gemfile setup for Travis by requiring version 2.2.5.

This commit updates the Travis version of Rubinius to 2.2.5+, which
includes the ruby standard library. Thus platform-specific Gemfile
configuration is no longer needed for Rubinius.

Also, Rubinius 2.2.5 does not work with sqilte3 1.3.9, so require
sqlite3 version 1.3.8 until it does.

💻 To run the tests locally, you will need to use Rubinius
2.2.5.

Gemfile

@@ -3,7 +3,9 @@ source 'https://rubygems.org'
 gemspec
 
 platforms :ruby do
-  gem 'sqlite3'
+  # sqlite3 1.3.9 does not work with rubinius 2.2.5:
+  # https://github.com/sparklemotion/sqlite3-ruby/issues/122
+  gem 'sqlite3', '1.3.8'
 end
 
 platforms :mri do
@@ -15,10 +17,4 @@ platforms :jruby do
   gem 'activerecord-jdbcsqlite3-adapter'
 end
 
-platforms :rbx do
-  gem 'json'
-  gem 'rubysl', '~> 2.0'
-  gem 'racc', '~> 1.4.10'
-end
-
 gem 'rails', '~> 4.0.0'