Specify ruby version and use bundler to fix gem versions

[Liquidsoul]

Checklist

• [x] I've checked that all new and existing tests pass
• [x] I've updated the documentation if necessary
• [ ] I've added an entry in the CHANGELOG to credit myself

Description

• add a .ruby-version to specify the ruby version to use.
• add a Gemfile to specify the version of the gem to use, mostly for rake and cocoapods.

Motivation and Context

When developing on the project, it can be difficult to ensure a consistent environment when the tools used or their versions are not specified somewhere.
This PR tries to help by specifying the ruby environment that should be used when running the various tasks.

[AliSoftware]

Done some similar work in https://github.com/AliSoftware/OHHTTPStubs/pull/368 just now