mono 3.8.0

Updates the mono formula to the 3.8.0 release, which contains a fix for the libMonoPosixHelper.dylib path issue.

I've tested all formulas that depend on mono (`brew uses mono`):

 - fsharp: Installed and ran the test included in the formula, which worked fine.
 - nant: Same as fsharp, no problems here either.
 - mercury: Installed with `brew install --with-mono mercury` and tested the installation by running `mmc --make --csharp hello` in the source distribution's sample directory. This generated a working binary.