Use after_failure not on_failure

Travis documentation wants the key after_failure not on_failure:
http://docs.travis-ci.com/user/build-configuration/

This key has been updated at craigcitro/r-travis:
https://github.com/craigcitro/r-travis/blob/master/sample.travis.yml
Merge pull request #617 from lmullen/update-travis-template

Use after_failure not on_failure