Adds the ability to remove a symlinked sdk from the command line.

Typical usage would be `fix-xcode remove 7` to make it so that Xcode only sees the iOS 6 SDK on the next restart. Running fix-xcode will restore the 7 SDK back. You can pass 7 or 7.0

Please note that I just ported this over from my script changes and didn't actually TEST this in the gem form. Might want to do that before merging in, @jamonholmgren.