git-delete-merged-branches: use -D instead of delete-branch

too slow
Merge branch 'master' of github.com:visionmedia/git-extras
Merge branch 'master' of github.com:visionmedia/git-extras
Merge branch 'master' of github.com:visionmedia/git-extras
Merge branch 'master' of github.com:visionmedia/git-extras
move docs target up
Release 1.8.0