Fix 404 Mozilla Aurora

Latest Mozilla Aurora is 24 now
Update Virtualbox to 4.2.16

Also moves the version-specific details out of the code into attributes
Use rbenv compatible ruby version for foodcritic
Add shasum tip for dmg_package
Merge pull request #64 from karlfreeman/patch-1

Fix 404 Mozilla Aurora
Merge pull request #65 from miketheman/update_virtualbox

Update Virtualbox to 4.2.16