In working out upgrading our Mediawiki deployment to Ubuntu 16.04
LTS (Xenial Xerus), it became apparent that libapache2-mod-php5 is
not available there because the distro has moved on to
libapache2-mod-php7.0 instead. By using the libapache2-mod-php
virtual package name instead, we can support both and also future
version bumps as well.
Change-Id: I5d1a5e51beb09a1cb26b1de6dba1b7d05781ee03