Changed travis.yml to not execute tests if compiling failed

According to 
* http://docs.travis-ci.com/user/build-lifecycle/#Breaking-the-Build
Travis will execute all steps in the "script" section even if one of them fails.

In order to save time on travis, I changed the build config so that "script" only compiles and the testing is done in "script_after".  This should prevent the tests from being executed if the build failed.