Upgrade Ruby version constraint to 2.x series

1.9.3 is annoying in that it requires explicit UTF-8 encoding headers in
source code files. Let's move into the future by not supporting that.
Add Rubocop configuration

When fixing all Rubocop warnings in fefc82b we didn't realize that a
Rubocop configuration in our home directory was being used.

Add a Rubocop configuration to this repository itself.