Test in several versions of ActiveRecord

Note that tests fail in AR 3.2.12
Make "spec" the default rake task

Travis uses the default task to try to run tests
Fix environment variable communication
Protect against empty env variable
Use travis to run build matrix
Remove check for 1.8.7, it is not compatible
Merge pull request #26 from begriffs/master

Run tests in AR 3.0.0 and 3.2.12