Rubygems on Ubuntu seems to break itself when you update:
sudo gem update --system
I've lost track of the number of times I've had to fix this (and inevitably forgotten how to do it each time) so for anyone interested here is the fix:
* Open a terminal
* sudo vi `which gem`
* under the line:
I can't remember where I first saw this fix, but I know I've found it here at least once, so thanks for the help Nick.