When pushing, delete branch in origin as well

When finishing a release while specifying the -p flag, the release branch is removed on origin.  For hotfix, it did not, so this change makes behavior consistent between both functions.

See https://github.com/jhoerr/posh-gitflow/blob/master/GitFlowExtensions/git-flow-release#L287-L315 for reference for how this works in the release workflow.