Bare minimum to make project work

[adnikiforov]

Basically that is changes I done to make project work in development and production modes on Ruby 2.6:

• pin pg and rake gems due to deprecations
• remove capybara-webkit, using selenium-webdriver instead (forget about capybara-webkit, it is dead)
• add monkeypatch to prevent threading issues with specs
• add assets workaround to make it work in development mode

I think we can create release 1.0.0 (stable) after merging this in master, and from stable we can move forward. BTW specs are broken anyway, but project itself seems working fine.

[adnikiforov]

Added instruction based on steps I did to run project locally: https://github.com/phusion/juvia/wiki/Development