Ruby 2.7.x support (update `json` to 2.3.x)

[BanzaiMan]

I've added testing with Ruby 2.7.x (https://github.com/travis-ci/dpl/commit/f369d65fea3c2d874fcefad79a8c1273d95cb294) but it fails (e.g., https://travis-ci.com/github/travis-ci/dpl/jobs/440752783).

The cause appears to be the outdated json/json_pure, but the attempt to upgrade it would fail:

$ bundle update
Fetching gem metadata from https://rubygems.org/........
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Bundler could not find compatible versions for gem "faraday":
  In Gemfile:
    faraday (~> 0.9.2)

    ey-core (~> 3.5) was resolved to 3.6.1, which depends on
      faraday (~> 1.0.1)

Bundler could not find compatible versions for gem "json":
  In Gemfile:
    json (>= 2.3.0)

    ey-core (~> 3.5) was resolved to 3.5.0, which depends on
      json (< 2.0)

[stale[bot]]

Thanks for contributing to this issue. As it has been 90 days since the last activity, we are automatically closing the issue. This is often because the request was already solved in some way and it just wasn't updated or it's no longer applicable. If that's not the case, please do feel free to either reopen this issue or open a new one. We'll gladly take a look again! You can read more here: https://blog.travis-ci.com/2018-03-09-closing-old-issues

[SleeplessByte]

Not stale