Added HHVM and PHP 5.6 to travis-ci

run build against PHP 5.6 and hhvm but allow them to fail to just see how it works.

http://docs.travis-ci.com/user/build-configuration/#Rows-That-are-Allowed-To-Fail

- also skip disabling xdebug on HHVM
- execute sh with `-e` to break on first error