Some additional improvements to the scaffolder

1) specifying `--license NONE` or not specifying a license used to result in a LICENSE.txt file with the MIT license in it (created by `bundle gem`--this file now gets deleted
2) regardless of the users license choice -- the generated gemspec still had `spec.license = "MIT"` in it (generated by `bundle gem`)--now corresponds to the selected license
3) `bundle gem` stages all of the scaffolding it generated. Since methadone does things on top of that, this results in a mixtures of staged and unstaged files--now everything gets staged.

Each change is self-contained.
Can you use `sh` here? (it's provided by `Methadone::SH`).  It's more cross platform.  Also handles error cases e.g. if `git` were to fail, we'd want to blow up here, too.