https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-4-4-released/

> This release includes some bug fixes and some security fixes.
>
> - CVE-2017-17742: HTTP response splitting in WEBrick
> - CVE-2018-6914: Unintentional file and directory creation with directory traversal in tempfile and tmpdir
> - CVE-2018-8777: DoS by large request in WEBrick
> - CVE-2018-8778: Buffer under-read in String#unpack
> - CVE-2018-8779: Unintentional socket creation by poisoned NUL byte in UNIXServer and UNIXSocket
> - CVE-2018-8780: Unintentional directory traversal by poisoned NUL byte in Dir
> - Multiple vulnerabilities in RubyGems

* 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

* Update rspec-rails to version 3.5.2

* Update addressable to version 2.5.1

* Update autoprefixer-rails to version 6.7.7.1

* Update bullet to version 5.5.1

* Update domain_name to version 0.5.20170404

* Update letter_opener_web to version 1.3.1

* Upate redis-rails to version 5.0.2

* Update active_record_query_trace to version 1.5.4

* Update capistrano-rails to version 1.2.3

* Update dotenv-rails to version 2.2.0

* Update pg to version 0.20.0

* Update tilt to version 2.0.7

* Update warden to version 1.2.7

* Update tins to version 1.13.2

* Update terminal-table to version 1.7.3

* Update oj to version 2.18.5

* Update simplecov to version 0.14.1

* Update uglifier to version 3.1.13

* Update hashdiff to version 0.3.2

* Update webmock to version 2.3.2

* Update devise to version 4.2.1

* Use ruby version 2.4.1

* Update sass to version 3.4.23

* Update puma to version 3.8.2

* Update will_paginate to version 3.1.5

* Update font-awesome-rails to version 4.7.0.1

* Update fuubar to version 2.2.0

* Update pry-rails to version 0.3.6

* Update simple-navigation to version 4.0.5

* Update rubocop to version 0.48.1

* Update doorkeeper to version 4.2.5

* Update faker to version 1.7.3

* Update aws-sdk to version 2.9.5

* Update fabrication to version 2.16.1

* Update hamlit-rails to version 0.2.0

* Update http to version 2.2.1

* Update httplog to version 0.99.2

* Update sidekiq to version 4.2.10

* Update rspec-sidekiq to version 3.0.0

* Update pghero to version 1.6.4

* Update rack-cors to version 0.4.1

* Update i18n-tasks to version 0.9.13

* Update ruby-oembed to version 0.12.0

* Update jquery-rails to version 4.3.1

* Update simple_form to version 3.4.0

* Update react-rails to version 1.11.0

* Update aws-sdk to version 2.9.6

* Update sidekiq-unique-jobs to version 5.0.0

* Update uglifier to version 3.2.0