Revision support doesn't work, make the formula head only

homebrew's BazaarDownloadStrategy doesn't actually support the `:revision` argument -- check out the code:

https://github.com/mxcl/homebrew/blob/master/Library/Homebrew/download_strategy.rb#L596

...so this formula actually always builds HEAD. Until someone adds this feature to homebrew, I thought the easiest solution was just to make this formula HEAD-only. Since you seem to want to keep this formula up-to-date with the latest revision anyway, this should save you some effort, too.