Upgrade bundler to 2.x

[artur-intech]

https://bundler.io/v2.0/guides/bundler_2_upgrade.html

Version autoswitching means that Bundler will automatically run the correct 1.x or 2.x version based on your lockfile

Existing app cannot be updated via bundle update --bundler, since Rails 4 supports Ruby >= 1.9.3, whereas bundler 2.x does not support Ruby < 2.3.
https://github.com/rails/rails/blob/v4.2.11/rails.gemspec#L10
rails/rails#34925

Rails 5 does not have such limitation.
https://github.com/rails/rails/blob/master/rails.gemspec#L36

---

Run on every deployment:

1. gem update --system
   Updates RubyGems
2. gem install bundler
   Updates bundler

---

Blocked by #377

[teadur]

@artur-beljajev Tests seem to run fine https://travis-ci.org/internetee/registry/builds/580155989

[artur-intech]

#1275 (comment)

[teadur]

Fixed by #1280