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
I suggest using ``[ '$TRAVIS_PHP_VERSION' != 'hhvm' ]`` rather than ``! [ '$TRAVIS_PHP_VERSION' = 'hhvm' ]`` to check whether it works better