Add --prefix to setup.py invocation

I got this error when virtualenv-burrito.py tried to install virtualenv:
`error: must supply either home or prefix/exec-prefix -- not both`

which led to the installation failing with this error:
`IOError: [Errno 2] No such file or directory: /Users/td/.venvburrito/bin/virtualenv`

Using an empty `--prefix` argument to setup.py, as suggested in [this blog post](http://biocs.tumblr.com/post/347221358/if-you-get-this-error-when-trying-to-use-the-home) solved the issue.