Bump ruby version to 2.4.2 (#4958)

* Bump ruby version to 2.4.2

https://www.ruby-lang.org/en/news/2017/09/14/ruby-2-4-2-released/

Gemfile.lock is also updated.

TODO
- [ ] Update Dockerfile with Alpine release of ruby-2.4.2

* Revert jwt version

It seems that jwt 2.0.0 fails in some environment.
ref. https://github.com/zaru/webpush/issues/42

* Bump Ruby version on docker image