Fix travis build

[arturcic]

Fixed the travis build. travis build

Appveyour works as well appveyour build

[dazinator]

Have taken a look. With the change to msbuild does this mean that we are back to using mono on linux in order to build? Or does msbuild on linux use dotnet core? Wondering if mono can be removed :-)

[dazinator]

I think it's probably safe to merge this given you have fixed the broken travis build and added the test filter! We can work out the mono dependency stuff seperately later.

[arturcic]

It's using the msbuild on each platform

https://github.com/cake-build/cake/blob/develop/src/Cake.Common/Tools/MSBuild/MSBuildResolver.cs

[dazinator]

Thanks - so looking at that it does seem to depend on mono: https://github.com/cake-build/cake/blob/develop/src/Cake.Common/Tools/MSBuild/MSBuildResolver.cs#L17

Msbuild is also available under .net core now: https://github.com/Microsoft/msbuild which means a mono install would be unnecessary - something we can look at seperately later.