Updated runtime dependencies to current production versions

The gemspec's runtime dependencies use current versions, based on Gemnasium's analysis...

https://gemnasium.com/ivanoblomov/goodreads

You may want to add the main repo too.

Since we're reasonably sure that Goodreads works with the current versions or earlier, maybe the version operator should be "<=" instead? Although "~>" seems to be more popular, I'm not really sure what's best practice here. Thoughts?