Update ruby-twitter to 5.7.1.

Changelog (from CHANGELOG.md)

5.7.1
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* [Only warn if entities are not included](https://github.com/sferik/twitter/commit/82ed19e69c03ccaccc366744421b3bb3ee444625) ([@matthewrudy](https://twitter.com/matthewrudy))
* [Fix typos in documentation](https://github.com/sferik/twitter/pull/531) ([@attilaolah](https://twitter.com/attilaolah))

5.7.0
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* [Remove `Twitter::Base.from_response` method](https://github.com/sferik/twitter/commit/6f9a352adaa5eee0611fa8d2e45fc7349b5cecae)
* [Remove `Twitter::REST::API` namespace](https://github.com/sferik/twitter/commit/f9c4e8214bfe0e4cbd13681a6454956f2c6ac21b)
* [Remove `descendants_tracker`](https://github.com/sferik/twitter/commit/4cbcb0fc58c55b84b642bc7c66085bb581e9b20a)
* [Remove unused `HTTP_STATUS_CODE` constants](https://github.com/sferik/twitter/commit/b45d89bc96f11079b31976f14ade4f89d50b4dc9)
* [Remove `Twitter::Error#cause`](https://github.com/sferik/twitter/commit/a5748b925aa3220e7388311bd0228a62d45d09a4)

5.6.0
-----
* [Replace custom `Twitter::NullObject` with `Naught`](https://github.com/sferik/twitter/commit/bc3990e3588f889569f1c92e465c329c508ce32e)
* [Use `URI` and `CGI` to convert query string into a hash](https://github.com/sferik/twitter/commit/6dd9d97aaef3917868b870e01896ab937cbacfbb)
* [Increase default timeout values](https://github.com/sferik/twitter/commit/350536926d1c2c0656fdc79948c5f543e306e14e)
* [Add `Twitter::Error::RequestTimeout`](https://github.com/sferik/twitter/commit/3179537af41b2e2f85c0bc74be799faea0817e48)
* [Remove unused methods `#put` and `#delete` in `Twitter::REST::Client`](https://github.com/sferik/twitter/commit/eaaf234b84a6e2e1f7695f73befaf11f30664f4a)
* [Deprecate `retweeters_count` in favor of `retweet_count`](https://github.com/sferik/twitter/commit/ea39bd013364dc24f02e9a1eb25b4b48b99a5480)
* [Deprecate `favorites_count`/`favoriters_count` in favor of `favorite_count`](https://github.com/sferik/twitter/commit/ea39bd013364dc24f02e9a1eb25b4b48b99a5480)
* [Deprecate `to_hash`/`to_hsh` in favor of `to_h` or `attrs`](https://github.com/sferik/twitter/commit/63e2cdd693aece2912564988d06786cd23a2cad5)
* [Deprecate `oauth_token` and `oauth_token_secret` accessors](https://github.com/sferik/twitter/commit/ac246717557ea02565a1d93b3f9a975e7fd39874)

(tsutsui)