Code Debt

This release mainly refactors the code, using rubocop to update the code style from 2014 conventions to 2018 conventions.

Bugfixes

• If a cell for a known column is nil, return nil instead of raised "undefined method"

Breaking Changes

• Bumps the minimum required ruby version from 2.1.1 to 2.4.4, the (currently) oldest ruby version still being supported
• Config::FileDefinition#output_map: The optional default parameter is now the last parameter, instead of the first