gitx-rowanj: Version and checksum

According to #1021, versioned/checksummed downloads should be preferred when available and the URL contains the version number. This is the case with [gitx-rowanj](https://github.com/caskroom/homebrew-cask/blob/master/Casks/gitx-rowanj.rb)—in fact, the appcast referenced links to the versioned downloads, not the unversioned one.