Update dependencies and Rubocop configuration

[johanhalse]

Tolken's dependencies need a facelift! I've gone over and bundle updated the ones that needed updating. That was pretty easy, the tests all run fine.

Rubocop, however.

The old configuration was very prescriptive and onerous. I gave up on trying to get up to date with the twenty million new or renamed cops, and instead replaced the original .rubocop.yml with one that starts with the defaults and adds some tweaks to minimize unnecessary churn. Tests still work fine.

I've also bumped the postgres version used in Travis, and opted for 13.4 which is the latest in the 13 series since 14 was released just a few weeks ago. I changed from using an explicit rvm key on travis to using .ruby-version too, for greater compatibility.