Updates & Improvements

This is quite a big collection of improvements:
* Updates all gems to release versions
* Fix Rails 4 deprecations
* Switch from postgres to sqlite3 (easier setup for a demo app)
* Adds turbolinks and conditional get to show that it works fine with sync
* Add sync to projects list
* Handle project form via js
* Add validations for project and todo model
* Some styling fixes and better bootstrap integration

I can break this down into smaller PRs if needed.