Replace gitreceive by a git plugin.

This removes the need for two distinct users.
Git pushes and dokku commands are sent using the same user.

tests/test_deploy

@@ -11,11 +11,11 @@ git config user.name "Test Robot"
 git add .
 git commit -m 'initial commit'
 REPO="test-$(basename $APP)-$RANDOM"
-git remote add target git@$TARGET:$REPO
+git remote add target dokku@$TARGET:$REPO
 git push target master
 URL=$(ssh dokku@$TARGET url $REPO)$FORWARDED_PORT
 sleep 2
 ./check_deploy $URL && echo "-----> Deploy success!" || {
   sleep 4
   ./check_deploy $URL && echo "-----> Deploy success!"
-}
+}