Increase Faraday dependency's maximum version to 3.0.0

[hmnhf]

Faraday has deprecated its middleware gem since version 2.0.0 and introduced a new built-in JSON middleware. (Which is the only middleware used by rubycent.)

This pull request:

• Updates Faraday's dependency version to allow using the latest version.

• Requires the middleware gem only if Faraday's version is older than 2.0.0. (The middleware gem can be removed in the future after changing Faraday dependency's minimum version to 2.0.0.)

• Fixes a failing test with Faraday versions 2.x. See the commit message of https://github.com/centrifugal/rubycent/commit/450ed404b7dda244106e2287d1e1153d79a5fb36 for a detailed explanation.

[FZambia]

@hmnhf hello, many thanks for the PR!

I fixed CI issue in https://github.com/centrifugal/rubycent/commit/060f20d28cecd427f33fe9f200d36ce4ddd8f631 - please rebase to the latest master.

@prikha Sergey, possibly you could check this out too?

[hmnhf]

@FZambia My pleasure. Rebased the branch with master.

[FZambia]

@hmnhf hello, part of v2.1.0 - thanks!