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
yes it should, as it is what is needed to make it install on 5.6. See the changelog: http://pecl.php.net/package-changelog.php?package=gmagick&release=1.1.7RC2