Typo BUNDLER_GEMFILE -> BUNDLE_GEMFILE

I was wondering why puma was logging that I wasn't using puma via bundle exec when it was the case. Turns out `ENV['BUNDLER_GEMFILE']` does not exists, `ENV['BUNDLE_GEMFILE']` does: 

`ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../../Gemfile', __FILE__)`