Remove sudo from gem installation instructions

Is there any good reason to be recommending users install with sudo?

Everything seems to work sans sudo, and since gems can execute arbitrary code, installing with sudo [can get dangerous](https://github.com/wmorgan/killergem) if anyone manages to hijack this gem or one if it's dependencies on RubyGems.

If a user _wants_ to install with sudo, they certainly can, and then they are assuming all the risk by being explicit. I don't think that we should be actively recommending it.