.travis.yml: rbx version is changed to 2.2.3

 * rbx version is changed to 2.2.3
 * add dependent gems for rbx platforms, described in
   http://docs.travis-ci.com/user/languages/ruby/
.travis.yml: workaround to avoid error in Ruby 1.8.7 and jruby-18mode

 * workaround to avoid error in Ruby 1.8.7 and jruby-18mode
   (reference: https://github.com/rubygems/rubygems/pull/763 )