Add a `rake test` task and Travis config.

This setup allows you to run the unit tests [on TravisCI](https://travis-ci.org/alloy/libPusher/builds/4440510).

The hash syntax changes were needed because the available 1.9.3 version (RVM) is somewhat broken, so this uses the system Ruby (1.8.7).

In the future the test runner + formatter could probably slim down the output even more and add a dot progress formatter.