Drop old mono versions since they can't build net46

bording · bording · commit 68a4d775e857 · 2017-04-01T17:21:45.000-04:00
Use latest instead
diff --git a/.travis.yml b/.travis.yml
@@ -3,16 +3,12 @@
 
 language: csharp
 mono:
- - 3.12.0
- - 4.2.3
+ - latest
 
 os:
  - osx
  - linux
 
-# The newer image cannot find mono 3.12 in the PATH
-osx_image: xcode6.4
-
 env:
  global:
   - MONO_OPTIONS=--debug
