Force deletion of the feature branch after finish.

This fix also works when feature branches are created manually, based on remote
(i.e. other developers) feature branches, to work on features together.

`git branch -d` won't remove local feature branches that are set up to track
remote branches, `-D` will.

Fixes ticket #31.
diff --git a/git-flow-feature b/git-flow-feature
index 618d7f1..d37857b 100644
--- a/git-flow-feature
+++ b/git-flow-feature
@@ -320,7 +320,7 @@
 	if flag fetch; then
 		git push "$ORIGIN" ":refs/heads/$BRANCH"
 	fi
-	git branch -d "$BRANCH"
+	git branch -D "$BRANCH"
 
 	echo
 	echo "Summary of actions:"