sferik@b40e208 changed the location of BASE_URL from Twitter::REST::Client to Twitter::REST::Request, but that change happened in version 6.2.0 of the twitter gem, so the dependency here needs to be bumped

3.1.0 of webmock adds http.rb 3.0.0 compatibility (see https://github.com/bblimke/webmock/blob/master/CHANGELOG.md#310), and twitter gem version 6.2.0 bumps http.rb version to ~> 3.0 (see https://rubygems.org/gems/twitter/versions/6.2.0)

As of sferik/twitter-ruby@e1e0095#diff-5ffd489b9cb331064f65077d37562b9c, the twitter gem no longer uses the deprecated `update_with_media` endpoint — it now calls upload followed by update with the previously uploaded media IDs.

The twitter gem depends on the `media_id` key that’s returned as part of the upload request, so this adds a fixture that includes it.

Before this change, these tests were throwing the following error:

```
     TypeError:
       no implicit conversion of Symbol into Integer
     # /home/travis/.rvm/gems/ruby-2.3.3/gems/twitter-6.2.0/lib/twitter/rest/tweets.rb:227:in `[]'
     # /home/travis/.rvm/gems/ruby-2.3.3/gems/twitter-6.2.0/lib/twitter/rest/tweets.rb:227:in `block in update_with_media'
     # /home/travis/.rvm/gems/ruby-2.3.3/gems/twitter-6.2.0/lib/twitter/utils.rb:23:in `block in pmap'
     # /home/travis/.rvm/gems/ruby-2.3.3/gems/twitter-6.2.0/lib/twitter/utils.rb:23:in `collect'
     # /home/travis/.rvm/gems/ruby-2.3.3/gems/twitter-6.2.0/lib/twitter/utils.rb:23:in `pmap'
     # /home/travis/.rvm/gems/ruby-2.3.3/gems/twitter-6.2.0/lib/twitter/rest/tweets.rb:226:in `update_with_media'
     # ./lib/t/cli.rb:612:in `reply'
     # ./spec/cli_spec.rb:2562:in `block (4 levels) in <top (required)>'
```

 Conflicts:
	t.gemspec