Fully qualify reference to hhvm class

Use '::hhvm' instead of 'hhvm'. Without this, puppet fails on
deployment-apache01 with this error:

    err: Could not retrieve catalog from remote server: Error 400 on
    SERVER: Duplicate definition: Class[Applicationserver::Hhvm] is
    already defined; cannot redefine at
    /etc/puppet/modules/applicationserver/manifests/hhvm.pp:17 on node
    i-0000011b.eqiad.wmflabs

Change-Id: I9077e448b21cdd804ff5896af78d4c6a5d468a35