[release-1.2] Use a more RFC2396 complaint regex pattern for the :url format validation

The current format's url regular expression pattern was tripping on a number of valid URLs, so this change updates it to use `URI.regexp`, which is provided by the ruby stdlib. To maintain some compatibility with the old regex, I have limited the valid urls accepted by this regex to only the `http` and `https` schemes, which may or may not be desired.