Refactor cross rubies

This fixes the following problems with the current cross-build scripts, `Rakefile` and `build_all`.

- Invocations of `rake-compiler cross-ruby` will fail due to version mismatch
- Ruby 1.9.3 cannot be cross-built with make -j2, which is the default MAKE command of rake-compiler-dev-box.
- Duplicates and hardcoded ruby versions and arch/platform names should really be reduced.
- Maintaining the list of cross-rubies should be made easier.

With this PR merged, `./build_all` should pass without a problem on a rake-compiler-dev-box environment except one problem which should be addressed in luislavena/mini_portile#30.

Reviews are much appreciated. /cc @larskanis, @flavorjones