Remove ffi conditional install on Windows platforms

[sds]

This functionality has caused more problems than it's solved.

From the original issue in #158, we're removing this logic for the following reasons:

1. Regardless of platform, you always see a Building native extensions. This could take a while... message when this is not actually the case. ChildProcess is just piggybacking off of the RubyGems extension facilities to conditionally install a gem. This is confusing.
2. When installing ffi via this workaround, it isn't clear to the user that it is happening. You see the aforementioned Building native extensions message, but aren't told that the ffi gem was installed. Again, this is potentially confusing, but also deceptive in that it hides the dependency.
3. The workaround requires us to specify a dependency constraint that doesn't respect the constraints defined in the user's own gem dependencies. Users then need to align their dependency constraints with what we've defined in mkrf_conf.rb, duplicating them in their Gemfile/etc. See cannot load such file -- ffi on windows #150 for an example where this confusion occurs.
4. It introduces a dependency on the rake gem (see Fix #143 - Childprocess v1.0.0 failing to install. #144), unnecessarily coupling us to Rake's release cycle. See Relax rake gem constraint from <=11.x to <=12.x. #147 for an example where we had to cut a new release when Rake issued a new release.

Given the changes to the behavior, this necessitates a major version bump from 2 to 3.