Fix warnings in GitBucketLinkAnnotatorTest

I removed AbstractProject/Build and started using FreeStyleProject/Build directly.

I know about the [formatting issue](https://github.com/ArloL/gitbucket-plugin/commit/8e02058e3418ea673ab04d58ee926f4d6641d5b6#diff-01ec07242da121849b2dc1469560c22bR61) and will it fix it in a seperate PR in which I will run Eclipse's Cleanup on the project.