Get dotnet core bindings working on Travis for macOS

[jbcoe]

Travis CI runs too old a version of macOS for the dotnet bindings from homebrew to work.

Once Travis CI supports the latest macOS this issue can be easily fixed by adding a command to install dotnet core to the macOS parts of .travis.yml

Update:

#299 has the .travis.yml changes needed.

Update

#299 does not lead to travis running dotnet on macOS

dotnet-sdk is the recipe needed but installing it via homebrew needs password entry. It's simpler to download the package and install it with sudo.