Require Bundler (a Gemfile) for all set up Middleman projects

Require Bundler (a Gemfile) for all set up Middleman projects. We still do extension auto-discovery for "init". Gemfile may now be in any parent directory of 'config.rb', in case the Middleman project is in a subdirectory of a larger project.

Also, remove the 'middleman upgrade' and 'middleman install' proxy commands - people should just use the Bundler CLI.