Travis CI for ActiveHash + Ruby 2, 1.8.7, Rubinius and JRuby support

As you can see, this is a big pull request.  When I was working on https://github.com/zilkey/active_hash/pull/66, I found it almost impossible to know if the work I was doing was breaking any specific Ruby or ActiveRecord versions because there is no reliable CI environment.  As such, I added support for Travis CI and ran it against my fork at https://github.com/ably-forks/active_hash/tree/travis-ci.

I found out very quickly that:

* Support for Ruby 1.8.7 was broken with the recent Aliases pull request https://github.com/zilkey/active_hash/pull/64
* Some simple tweaks were need to support for Ruby 2 
* Tests would not run in JRuby or Rubinius and some tweaking was required to get active_hash working in these environments.

I know I have pull rights to this repo, but I would feel better if you look at the pull request before accepting it.  Also, before this pull request will work, you will need to do the following:

* Follow steps 1 & 2 at http://about.travis-ci.org/docs/user/getting-started/ to get your active_hash repo linked to Travis CI
* Then just merge the pull request and the build should kick off immediately, and hopefully everything should pass.
* You should probably bump the version number and release a new version of the Gem.

Once Travis is setup, it should also now run tests against pull requests so at least you'll know if a pull request passes in all environments or not.

Shout if you need any help or want to chat this through.

Matt