Update GitLab CI configuration for NuGet deployments

* NuGet deployments are only done for tagged builds now
* Fix unnecessary using statement

.gitlab-ci.yml

@@ -36,5 +36,4 @@ deploy:nuget:
   stage: deploy
   script: dotnet nuget push ./Terminal/bin/Debug/*.nupkg -k $NUGET_API_KEY -s https://www.nuget.org/api/v2/package
   only:
-    refs:
-      - master
+    - tags

Terminal/Commands/ListCommand.cs

-using System.Collections.Generic;
 using TBureck.Terminal.IO;
 
 namespace TBureck.Terminal.Commands