chore(release): be able to release any commit

The version information is now stored only in the tags.
By this we are able to release commits in the past, which
have already been tested, so we don't need a code freeze
or run tests any more. This is also the first step for
letting Travis do the releases in the future.

The package.json now contains the new
property 'branchVersion' that defines which tags are
valid on this branch.

Closes #6116