skipping the firefox version validation if firefox is not the browser be...

...ing used


@jzoldak @benpatterson 

This makes it so that a user doesn't have to use `--skip_firefox_version_validation` to get around the firefox version error if they are not even using firefox.  As in, if I'm running tests in chrome, I shouldn't get an error saying `Exception: Required firefox version not found.`

Bok Choy defaults is to firefox: https://github.com/edx/bok-choy/blob/a851075ceba9fc57e9b3ca8ee40816954c46e0fa/bok_choy/browser.py#L167