Reformat .travis.yml

Similar to https://github.com/silverstripe/silverstripe-framework/commit/3cfb2851227f7c9b68feef86a9b8de1da8bc5769#diff-d41d8cd98f00b204e9800998ecf8427e, but also adds hhvm-nightly as an allowed failure and promotes PHP 5.6 to the standard test suite.