make "git flow release" tag master

``git flow release`` was tagging the last commit in the release branch instead
of master.

This commit makes it tag master (after the merge) instead.

I believe this commit restores the original and intended behavior, which
appears to be changed (probably unintentionally) in commit afb191f7.
Merge pull request #1 from JasonWoof/feature/release-tags-master

merged "JasonWoof:feature/release-tags-master" into develop