Update Travis config to match supported versions

1. Drop support for Ruby < 2.0
   (https://www.ruby-lang.org/en/news/2014/01/10/ruby-1-9-3-will-end-on-2015/)
2. Drop support for Rails < 4 (http://rubyonrails.org/security)
3. Test against only stable versions of the 4.0.* and 4.1.* versions of
   Rails and master