Update rubocop configuration file to run locally

Created by: gracewashere

Problem:

Hound uses Rubocop under the hood to check for Ruby style violations. Its default config is different from Rubocop's defaults. This means that people who run rubocop locally will get different feedback than they get from Hound.

Solution:

Rename .ruby-style.yml to .rubocop.yml, so Rubocop automatically picks up the configuration when run locally.

Update the rubocop config to use thoughtbot's default.