Parameterise the xquartz class

Allows module users to specify their own version of xquartz without
having to bump the module version.
Bump to XQuartz 2.7.6 by default
Merge pull request #10 from boxen/upstream-2-7-6

Update to XQuartz 2.7.6
Update to XQuartz 2.7.7
Merge pull request #1 from tcooper/XQUARTZ-2.7.7

Update to Xquartz 2.7.7