Major Description
In AVRO-2112, we upgraded to the latest Mono v5+ to test the C# project in the Travis CI builds. We should test against the .NET Core SDK instead for the following reasons:
- .NET Core/Standard should become the default that we build and test against because that is quickly becoming the default across the community.
- Installing the latest Mono significantly increased Travis CI build times, as can be seen from the build history. Before build #317 (the one that merges the C# updates), builds were generally under 40m. Following builds regularly push the 50m Travis CI build limit. By switching to .NET Core SDK instead, build times drop back down to the normal before the C# updates (see this example).