Accepts strings for group names. Fixes #2305

This allow Environment#setup behaves in same way as
Environment#require, allowing usage of ENV variables directly:

  require "bundler"
  Bundler.setup(:default, ENV.fetch("RACK_ENV", "development"))
Merge pull request #2307 from luislavena/runtime-setup-strings

Accepts strings for group names
parse Rubygems cert failures