Try running unit tests on HHVM

[ptarjan]

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[stof]

I would mark it as allowed failure for now, to avoid having broken builds all the time until the HHVM support is working (otherwise people will take the habit to ignore the build status)

[Majkl578]

@stof: Agreed, just add the following:

matrix:
    allow_failures:
        - php: hhvm