Fix Rakefile so Travis test failures are properly reported
Modify Travis test procedures

This commit modified the travis test procedures so that we can have OS
X fail. Now, a build will finish as soon as a job has failed, or when
the only jobs left allow failures.
Merge pull request #22 from sbonami/22-Fix-travis.yml-so-failures-are-properly-reported

Fix Rakefile so Travis test failures are properly reported