Add gem.deps.rb ruby engine filtering

This should complete the major features of platforms filtering.
Use Gem.ruby_engine for ruby 1.8.7 support

It knows how to handle the missing RUBY_ENGINE constant.
PLATFORM_MAP should not be documented
The ruby platform is only for CRuby and Rubinius

This boggles my mind.  Why isn't JRuby a ruby?
Don't reuse sets across platform tests

This fixes a test bug.
Omit nil platforms in ENGINE_MAP

This reduces the lines in the ENGINE_MAP and makes the code nicer.
Remove extra blank line
Add windows exclusion for gem.deps.rb platforms

The ruby, mri and rbx platforms are not such platforms on windows.
Add maglev platform to gem.deps.rb support

maglev is an undocumented platform supported by bundler.
Match gem.deps.rb platforms matrix with bundler

This implements the windows checks better by marking the platforms as
either only-windows or never-windows.  I'm thinking we're now fully
compatible with bundler.

See also bundler/bundler#2691 where I suggest an alternative to the
contradictory platforms that bundler has.