Ensure lib/library_web/controllers/plugs/require_auth.ex is covered by tests

I have created the lib/library_web/controllers/plugs/require_auth.ex plug so that we can protect routes by login, however in #38 it is not used anywhere, the easiest (and most effective, IMO) way to test a plug to check auth is to use the normal way to test a controller, but with both a user set on the connection, and no user on the connection, so you can ensure you know the results of both. Due to this I have added `lib/library_web/controllers/plugs/require_auth.ex` to the files igfnored by coveralls, but once we use it somewhere, we must ensure it is covered, and remove it from the list of ignored files.