Remove trailing slash from url to not delete CWD when attempting repository.remove() #99

This fixes the problem of removing the __current working directory__ when the testrun fails before the `mozmill-tests` repository has been properly cloned.

See issue #99

The issue here is that if we have a trailing slash in the url, instead of trying to delete _cwd + repository_name_ are are issuing the command to delete the _cwd_

I'm thinking we should revisit this and not even __try__ to delete the repository if it wasn't successfully cloned.  

@davehunt & @whimboo please have a look at this
I am fine with just this quick fix for now, but if you feel we should make more significant changes as mentioned in the previous paragraph, please let me know.

Thanks