More OSX launcher tweaks:

* Require a framework install of Mono, as our .config files reference dylibs installed to the frameworks dir.
* Bump the minimum Mono version to 3.2, which included the SGen GC.
* Update download link for the new mono website.
* Reference the MRE package to reduce potential confusion.
Merge pull request #6452 from pchote/more-osx-launcher

More OSX launcher tweaks