Add cake-addin tag to Cake.TravisCI NuGet package

The [NuGet Gallery](https://nuget.org) now includes a "Cake" tab with instructions on how to use NuGet packages in Cake build scripts. I'd like to suggest that you include the tag `cake-addin` in the NuGet package of this addin, so that the NuGet Gallery can display the correct instructions to install this addin.

The UI looks similar to this:


Documentation on best practices on tags for Cake addins: https://cakebuild.net/docs/extending/addins/best-practices#tags

Inspect the project’s NuGet package metadata and packaging configuration to find where package tags are defined. Add the `cake-addin` tag, then build or pack the project and verify that the generated package metadata includes it.

Written by the indexing model from the issue text.