Fix remote uniqueness validation because of double slash

This fixes an issue I had:

    Started GET "/uniqueness?case_sensitive=true&user%5Bemail%5D=beat.besmer%40gmail.com&_=1367362834432" for 127.0.0.1 at 2013-05-01 01:00:45 +0200

    ActionController::RoutingError (No route matches [GET] "/uniqueness"):

The problem was that the url would contain two slashes `//` (JS Console)

    JSCONSOLE> ClientSideValidations.remote_validators_url_for('uniqueness')
    "//localhost:3000//validators/uniqueness"

Also see <http://stackoverflow.com/questions/15903572/actioncontrollerroutingerror-no-route-matches-get-uniqueness>