(maint) Specify mono version for Travis-CI

This allows the build to compile but still fails on NUnit tests.

@sideeffffect determined in chocolateyGH-369 that there may be a bug in the most recent
mono and has filed an issue at [Xamarin Bugzilla #32561](https://bugzilla.xamarin.com/show_bug.cgi?id=32561).

Prefer 3.12.0 which gets the [latest in the 3.12.X line](travis-ci/travis-ci#4130).

Related Travis Doc - [Choosing mono version](http://docs.travis-ci.com/user/languages/csharp/#Choosing-Mono-version-to-test-against)

.travis.yml

@@ -1,4 +1,6 @@
 language: csharp
+mono:
+  - 3.12.0
 install:
   - sudo apt-get install mono-devel mono-gmcs
   - nuget restore src/chocolatey.sln