changed travis to run on the nightly builds of HHVM until everything gets stable

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

As HHVM is not stable enough yet to run Symfony, let's test on the HHVM nightly builds. It will allow us to get a better feeling of the remaining problems with HHVM. ping @deguif