Update Mono formula to v3.4.0 (with bugfix)

Updates the Mono formula from v3.2.8 to v3.4.0, including the backported bugfix mentioned here:
https://bugzilla.xamarin.com/show_bug.cgi?id=18690

Once a new 3.4.x tarball gets released, the bugfix can (and should) be removed from the formula.

Also note that I took the liberty of updating the SHA1 checksums to SHA256. If you're against that for whatever reason, let me know, and I'll downgrade to SHA1.