Update PHP PPA to old stable to continue installing PHP 5.4

The PPA that was in use switched to providing PHP 5.5 and introduced a new dependency that we were not able to provide immediately. This change switches to the old stable release and continues to use 5.4 until we have a chance to plan it out better.

Closes #110
Merge branch 'master' of github.com:10up/varying-vagrant-vagrants