updated Dropbox to version 2.0.5

Added md5, sha256 to cask

Tested on OSX 10.8.3

I'm a little new to Cask - looks awesome. I was curious about the behavior of updating/upgrading an existing package, such as Dropbox 2.0.

If 2.0 is installed in in `/usr/local/Caskroom/dropbox/2.0` and 2.0.5 gets installed in `/usr/local/Caskroom/dropbox/2.0.5` - then when `brew cask linkapps` runs, it does make two symlinks, just the second one clobbers the first (which is ok).

Is there a better way to do this, and clean up the older version? `brew cask uninstall dropbox` will only remove the current one, apparently.