Don't build release branch or tags.

@adamtheturtle I've tested the tags, but not the `release/` prefix.
So is `git tag -a $VERSION release/flocker-${VERSION%.*}` at https://github.com/ClusterHQ/flocker/pull/293/files saying "You have a release branch in the form `release/flocker-X.Y.Z` so tag it as `X.Y.Z` and then this is checking if whether a branch is in the form  `release/flocker-X.Y.Z` meaning it is a release branch or `X.Y.Z` meaning it is a tagged release?

Please add a comment to make this clearer.