[Maven Skin] fix broken GitHub ribbon

The Maven Skin at https://github.com/bloomreach-forge/forge-maven-skin/ is outdated; it's a fork of Apache Fluido Skin 1.6, see http://maven.apache.org/skins/maven-fluido-skin.

Currently the right top "Fork me on GitHub" ribbon is broken because the resource at returns a 403 Forbidden. Its URL is https://s3.amazonaws.com/github/ribbons/forkme_right_green_007200.png

Let's fix this, either by re-forking on latest Fluido or by something quicker.

Second, let's remove the link to developers.bloomreach.com because it will be decommissioned this year.