rake vendorise:gem[url, branch] doesn't seem to pull in branch properly

I was running the command

```
$ rake "vendorise:gem[git@github.com:some/repo.git, sometag]"
```

and it didn't seem to pull in the correct tag. Running the git subtree command the Arborist uses manually did it fine, so I imagine the issue is in the arguments parsing in the rake task.

Other than that it runs like a shiny new Mercedes-Benz! 
