Update .travis.yml

Author: dsyme

.travis.yml

@@ -15,12 +15,12 @@ install:
 script: 
   - ./build.sh NuGet
 
-# See bug "https://bugzilla.xamarin.com/show_bug.cgi?id=41914"
-# peverify failing on Mono latest with "condition `res != EINVAL' not met", on OSX
+# Fails on Travis Linux Mono latest (4.6.1) because FSharp.Core PCLs are not installed, see
+# https://github.com/fsharp/fsharp/issues/623#issuecomment-253165838
 
 matrix:
   allow_failures:
-    - os: osx
+    - os: linux
       mono: latest