Add cask for diffmerge 3.3.2

Add cask for diffmerge 3.3.2 (http://www.sourcegear.com/diffmerge)
Update Diffmerge cask to use new link syntax

Update Diffmerge cask to use new link syntax
Merge pull request #376 from ragusource/diffmerge

Add cask for diffmerge 3.3.2