Update rest-client version to support >2.0.0

Versions of Ubuntu 18.04 will run into the following issue because OpenSSL is too new:
https://github.com/rest-client/rest-client/pull/573
```
/ruby/2.5.0/gems/rest-client-1.8.0/lib/restclient/request.rb:163:in `fetch': key not found: :ciphers (KeyError)
```

Start by locating the project's rest-client dependency declaration and checking its current version constraint. Reproduce the Ubuntu 18.04/OpenSSL failure described in the issue, then verify that the updated dependency resolves the reported KeyError and that the existing test suite passes.

Written by the indexing model from the issue text.