Fix build on 1.9.2

I added to test if it was the problem on this commit https://github.com/fernandes/fog-xenserver/commit/9ff127569f4850dfbf3469c0f8413c3a0e94d534, turn is on gemspec https://github.com/fernandes/fog-xenserver/blob/master/fog-xenserver.gemspec#L25 ... Just kept away from Gemfile

btw, why on Gemfile 1.9.2+ is getting fog-core from github and 1.9.2- not? any special reason?