Inject /etc/hhvm/php.ini in PHP's configure for HHVM

Modify `/etc/hhvm/php.ini` before `Travis::Build::Script#paranoid_mode` disables `sudo`.

This fixes https://github.com/travis-ci/travis-ci/issues/2863 and https://github.com/travis-ci/travis-ci/issues/2523.
much prefer `$()` over backticks, and if we're discarding the stdout why not use `cat >> #{ini_file_path}` instead of `tee`?