More OSX launcher tweaks

![](https://cloud.githubusercontent.com/assets/167819/4262202/207d1b60-3b93-11e4-91e9-f17630f7a889.png)

* 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.

After this has been merged I will manually backport this, #6300, #6307, and #6438 to our release-20140722 OSX package to solve the current crash and startup perf issues.