Tolk compatibility
New version 1.6
Revert "Update compatible Rails versions"

This reverts commit b4aef4335b93b7286335c0cc081e9900a6811366.
Merge tag '1.6.0'
Use modern Travis badge format
Remove note about official repo location

The point has been made by now.
It also makes for a useless README excerpt (on GitHub mobile, etc.).
Move CI database setup to before_script
Remove unused development dependencies