[OHAI-584] Update systemu to 2.6.4 for Ruby 2.1.x optimization

Update systemu dependency to 2.6.4.

There are a couple of reasons for doing this:

    Fixes for bizarre hostnames whilst running on Windows
    Fixes for Ruby 2.1.x compatibility

Since ahoward doesn't provide actual tagged releases (ugh), you can read the patch here. It's not very big.

https://github.com/ahoward/systemu/compare/cb253a8bf213beea69f27418202e936a22d7308f...076ef54e839e7ea0a70126dfe1f6c0ef5ed30859

Specs all pass with this. https://travis-ci.org/juliandunn/ohai/builds/25521166