Stop HHVM build from hanging

Some of the extensions were still being installed under the HHVM build, which seemed to be hanging the build.

This PR changes the travis-init.sh script so that extensions are only installed when building against the regular PHP interpreter. I also expanded commands onto multiple lines for the sake of readability - `set -e` ensures that the whole script fails if one command fails so that the commands do not need to be strung together with `&&`.

The HHVM build itself fails with fatal errors, but at least it actually get to the phpunit stage again now.