tiger-vnc: re-adding formula with x11 dep dropped

The formula builds only the client (vncviewer)
which doesn't depend on x11

cf. 
-   http://librelist.com/browser/homebrew/2014/12/31/tiger-vnc-vs-x11/
-   https://github.com/Homebrew/homebrew-x11/pull/12
I feel a little nervous letting people opt out of this, to be honest, but I guess ` :recommended ` is okay.