require UserInteraction before including it

Commit 5db04b1c07fcd9ae9e38313e371dfa2e708ca3b4 started including
Gem::UserInteraction into Gem::ConfigFile, but didn't add a require.
The Bundler tests against rubygems master started failing as a result.
If this should be done a different way, just let me know.
Merge pull request #444 from indirect/master

Require UserInteraction before including it