append the .rb extension on Cask::Source::Path

Unless `.rb` is already present on the query string.  Failure
to append the extension caused confusing effects, including
selective failure of "brew cask list" when invoked from
`/usr/local/bin` working directory.  Fixes #4077 .
Credit to @mroth on the detective work.
Merge pull request #4360 from rolandwalker/extension_on_cask_source_path

append the .rb extension on Cask::Source::Path