Fix up Twitter client configuration

Hi Parker

I tried to run `ebooks twitter` and I got

    /Users/sam/.rvm/gems/ruby-2.1.5/gems/ebooks-0.1.0/lib/ebooks/twitter.rb:10:in `initialize': undefined method `configure' for Twitter:Module (NoMethodError)

The Twitter gem (since version 5, I guess) now does configuration differently, see https://github.com/sferik/twitter/blob/master/README.md#configuration

Seems to work just fine like this :)

Sam
Can you make this `"~> 5.0"`, please? Want to lock to version 5.x.