Make spec runnable on Rubinius

This PR includes a number of Travis related changes:

1. Added Rubinius gems to the Gemfile
2. Removed the Gemfile.lock so different VMs can load different gem sets
3. Remove the master branch only restriction from .travis.yml so it runs on PRs
4. Removed 1.8.7 as it's not compatible with the ActiveSupport 4.0.x that is loaded by bundle

It may be desirable to put 1.8.7 back into the mix, and introduce different ActiveSupport 3.2.x and ActiveSupport 4.x gemfiles for use by Travis, if it's desirable to run specs against both versions of ActiveSupport.

Travis specs run green with this set of changes, and users can run specs locally under rbx-2.2.1.