Log Message:
Update ruby-twitter to 5.15.0.

5.15.0
------
* [`NullObject#as_json` returns \ 
'null'](https://github.com/sferik/twitter/commit/2979e703c09a45f012cb2c5b2d6663bf1f4d3351) \ 
([@lukevmorris](https://twitter.com/lukevmorris))
* [Add methods to get to parameters of quoted \ 
tweet](https://github.com/sferik/twitter/commit/afd41a3e36cc94194a2110ba9adce13486ced9fd) \ 
([@couhie](https://twitter.com/couhie))
* [Add additional mime_types for multi-part \ 
upload](https://github.com/sferik/twitter/commit/947fcdc9f7348f267d74933ffa43d191cf248a9c)
* [Fix bug where flat_pmap can return \ 
nil](https://github.com/sferik/twitter/commit/e22a5601ec702632510b3e983e50929ceb334b95)
* [Add new error \ 
codes](https://github.com/sferik/twitter/commit/1ce6b2f02d0f5f78435ee898e8f5b6d3db18d6f1)