[HAT] Load mod_version on application servers

'HAT' stands for 'HHVM, Apache 2.4, and Trusty' (in the spirit of 'LAMP').
I'm going to start using this tag in HAT-migration commits as a useful flag for
reviewers.

This particular patch ensures mod_version is loaded on all application servers.
We already settled on this approach in I5abf01810, which ensured mod_version is
enabled by default via the apache module. But the application servers have a
custom apache2.conf that does not recurse into /etc/apache2/mods-enabled/*, so
it is necessary to do this here for now.

We need mod_version so that we can introduce <IfVersion >= 2.4> / <IfVersion <= 2.2>
configuration guards and thus make our Apache configs compatible with both 2.4
and 2.2

mod_version is part of apache2.2-bin and thus available on all application
servers. You can verify the first claim by running:

    dpkg -L apache2.2-bin | grep mod_version

You can verify that the module is present on all app servers by running the
following command on palladium:

    salt 'mw*' cmd.run 'test -f /usr/lib/apache2/modules/mod_version.so'\
      ' && echo OK || echo MOD_VERSION MISSING!'

Change-Id: I639d4d6dc424fc750325aee7c4a104eba48c1821