Switched to the latest at present stable Mono (4.2.1) for the OS X build.

We used to rely on the latest nightly build because it used to have the same name but this is no longer true.

Signed-off-by: Dimitar Dobrev <dpldobrev@protonmail.com>

.travis.yml

@@ -20,7 +20,7 @@ install:
   - if [ "$TRAVIS_OS_NAME" = "linux" ]; then sudo apt-get update -qq; fi
   - if [ "$TRAVIS_OS_NAME" = "linux" ]; then sudo apt-get install -y mono-devel; fi
   - if [ "$TRAVIS_OS_NAME" = "osx" ]; then brew install wget xz; fi
-  - if [ "$TRAVIS_OS_NAME" = "osx" ]; then wget -O mono.pkg http://download.mono-project.com/archive/nightly/macos-10-x86/mono-MDK.pkg; fi
+  - if [ "$TRAVIS_OS_NAME" = "osx" ]; then wget -O mono.pkg http://download.mono-project.com/archive/4.2.1/macos-10-x86/MonoFramework-MDK-4.2.1.102.macos10.xamarin.x86.pkg; fi
   - if [ "$TRAVIS_OS_NAME" = "osx" ]; then sudo installer -pkg "mono.pkg" -target /; fi
   - wget https://nuget.org/nuget.exe
   - mono nuget.exe install NUnit -Version 2.6.4 -OutputDirectory deps