ruby.devkit should use the Chocolatey binroot, not the C:\ drive

The first commit log has most of my thoughts: b108d9577011f7e0939d9a98361160363c8225ca. I should have committed along the way. I did approach this step-by-step, in a traditional refactoring way, so everything _should_ work. I tested a couple of scenarios (fresh install, restoring customizations, faked old installs, etc) on my machine, Windows 7 Pro SP1 x64.

If I presume too much, I'll send just a `Get-BinRoot` pull request.
I'm pretty sure Invoke-Expression is only needed to execute commands in strings or variables, `ruby` should interpret as an executable file without it.