Upgrade Mono for CI

Author: migueldeicaza

.travis.yml

@@ -11,6 +11,6 @@ script:
   - cp -R $TRAVIS_BUILD_DIR/native/*.dylib $TRAVIS_BUILD_DIR/tests/TensorFlowSharp.Tests.CSharp/bin/Release/
   - cd $TRAVIS_BUILD_DIR/packages/xunit.runner.console.2.2.0/tools
   - mono --arch=64 xunit.console.exe "$TRAVIS_BUILD_DIR/tests/TensorFlowSharp.Tests/bin/Release/TensorFlowSharp.Tests.dll" "$TRAVIS_BUILD_DIR/tests/TensorFlowSharp.Tests.CSharp/bin/Release/TensorFlowSharp.Tests.CSharp.dll"
-mono: "5.4.1"
+mono: "5.12.0"
 dotnet: "2.1.4"
 os: "osx"