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.
We can't use `sudo cat` to redirect, since the user `travis` does not have the write permission to `/etc/hhvm/php.ini`. Sending *that* output to `/dev/null` prevents the file content from leaking to the log.