Update ruby-twitter to 5.9.0.

Changelog (per CHANGELOG.md):

5.9.0
-----
* [Use expanded URIs when available]
* [Deprecate `Twitter::REST::Client#middleware=` and `#connection_options`]
* [Ensure predicate methods return `false` for `Twitter::NullObject`]
* [Make `Twitter::Place#id` attribute accessible]
* [Enable injection of custom TCP/SSL socket classes]
* [Deprecate predicate methods without question marks]
* [Deprecate `Twitter::Base#[]`]
* [Remove statement that TweetStream is not 2.0.0-compatible]
* [Dont allow unacceptable IO objects in `Twitter::REST::Client#update_with_media`]
* [Add support for new REST API endpoint for bulk lookup of Tweets by ID]
* [Make the streaming API raise exceptions for status codes]
* [Call GET users/show if screen name has already been fetched]
* [Add the ability to set user_agent and proxy]
* [Use (immutable) user ID instead of (mutable) screen name]
* [Implement mute functionality]

5.8.0
-----
* [Alias `Twitter::Tweet#reply?` to `Twitter::Tweet#in_reply_to_user_id?`]
* [Add `Twitter::Error::SSL_REQUIRED` error code]
* [`Twitter::Tweet#retweeted_status` always returns the original tweet, not the retweet]

(tsutsui)