Updated rubies tested by travis-ci.org

- For details on preinstalled Ruby versions see: http://rubies.travis-ci.org/
- Removed `ree` Ruby. `activesupport` v4.0.5 introduced dependency on ruby v1.9.3
- Removed `ruby-2.0.1`. Does not exist!
- Added more 2.x versions of Ruby.
There are no executables within this gem.
Do not lock gem to ruby platform.

When building a gem, it will lock it to the ruby platform building the gem, this would prevent use with another Ruby version (jruby, rbx etc...)
Merge pull request #4 from lantins/rubies-tested-by-travis

Rubies tested by travis + gemspec changes