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.`

what is the default if the user does not have the SELENIUM_BROWSER env var set? chrome?