Upgrade packer to 0.7.1
Merge pull request #6147 from whiteley/packer

Upgrade packer to 0.7.1
updated unity3d (4.5.4)
Merge pull request #6148 from vitorgalvao/update-unity3d

updated unity3d (4.5.4)
CONTRIBUTING.md: Adding unversioned Cask info

The docs seemed to suggest that if versioned downloads were available,
to prefer them over unversioned ones. Since that's incorrect presently,
I attempted to correct that (see #6126 for more details).
Merge pull request #6145 from treyharris/versioning-docfix

CONTRIBUTING.md: Adding unversioned Cask info
Changed Firefox to Firefox.app in CONTRIBUTING, for consistency with other examples.