Windows Path fixes

This is a fix for issues uncovered in #353 and builds on the work @lamont-granquist started in #354. 

Normally Windows treats `Path` and `PATH` equivalently and Ruby returns the same data when querying `ENV['Path']` or `ENV['PATH']`. BUT if you begin a build in a Unix-like environment (e.g. Cygwin) only `PATH` is respected so we can't assume Windows builders will always use a path key of `Path`.

/cc @lamont-granquist @opscode/release-engineers