Apache config for Wikivoyage using mod_proxy_fcgi

This patch preserves the behavior of existing app servers running Apache 2.2
and mod_php5 by enclosing the config for HHVM in a <IfDefine HHVM> guard.

Before I go ahead and submit a massive diff, I wanted to solicit reviews for a
single, focused example. I chose Wikivoyage because it is relatively new and
thus has fewer legacy URL patterns to support than some other projects.

Some specific things worth paying attention to:

* When it appears by itself, a RewriteRule w/the [P] flag does not create a
  worker object, so proxied requests are handled by the default workers, which
  do not use HTTP Keep-Alive or connection pooling. But in this case the worker
  created by the ProxyPass directive will be usable for requests which match
  the RewriteRules because it shares the same backend (scheme://host:port).

* Both *.php and *.hh (Hack) are proxied to HHVM. We don't use Hack anywhere at
  the moment and have no specific plans for doing so, but I think that if
  introducing Hack is made contingent on a big change to Apache configs it'll
  discourage folks from trying.

* Apache talks with the backend via TCP sockets rather than Unix domain sockets
  because support for the latter is incomplete in the version of Apache that
  ships with Trusty (2.4.7). We'd need 2.4.8+. Maybe later.

Tested with /etc/hhvm/php.ini containing:

  hhvm.server.type = fastcgi
  hhvm.server.port = 9000
  hhvm.server.source_root = /usr/local/apache/common/docroot

Change-Id: I8f456a74a9bfa504be2f82e14fa5dcde5fe7a3f8