Fully DSL support inside group

What do you think about this as a solution to https://github.com/thoughtbot/appraisal/issues/76? It seems to make sense, fixes my issue, and doesn't break any tests.

It just treats `group` as a nested Gemfile, so group is just a wrapper and `Appraisal::Gemfile` does all the work. If it makes sense you could do the same for platforms.