Use automatic NuGet package versioning

[dbrattli]

We need a way to auto-generate the version number when we push to master to avoid build breaks when pushing to Artifactory.

• https://stackoverflow.com/questions/43274254/setting-the-version-number-for-net-core-projects-csproj-not-json-projects
• https://stackoverflow.com/questions/43019832/auto-versioning-in-visual-studio-2017-net-core

[einarmo]

Is this what we want? I suggest we use tags, then test for tags when building. This requires us to specify a tag on any commits we want to push to artifactory, but it removes the dependency on the code.

[dbrattli]

Sound like a good idea. Let's try this.