Make Bundler warning free again

Bundler 1.7.2 is not warning-free. This fixes the two Ruby warnings in 1.7.2.

The `warning: private attribute?` one (fixed by declaring a public `attr_reader` and then privatising it) is irritating and will hopefully one day get removed, but it’s there for now.

Keeping Bundler warning-free is helpful for anyone who develops with warnings turned on.