Fix Rails 4 bugs

I like the concept of the gem, I think it makes a lot more sense than the others.

Here are my contributions:
* Fix Rails 4 hstore access (symbol-vs-string keys was a pain)
* Add Appraisals to test on Rails 3.2, 4.0, 4.1
* Add Travis file (see https://travis-ci.org/joallard/trasto/builds)
* Fix exception on empty record

A bunch of cleanups and we're all set. I think a minor version increment is warranted. Cheers!