Removed dependence on environment variables by allowing variables to be set on instantiation

[itsterry]

I've made it so that you can specify client_id, client_secret, and redirect_uri at the same time as auth_token, like this:

client = InstagramBasicDisplay::Client.new(auth_token: token, 
                                           client_id: client_id, 
                                           client_secret: client_secret, 
                                           redirect_uri: redirect_uri)

This allows people to store their credentials in any of the other secure methods (Rails.application.credentials, etc)

I've also removed the bundler versioning restriction and the Ruby version restriction (they were getting a little old)

[yshmarov]

really great addition! I really prefer using rails credentials VS env vars.