Update code style

[dankimio]

I've used the default style guide from RuboCop.

This sets foundation for further changes from #56.

Changes

• Removed Ruby 2.3 safe navigation syntax

[danrabinowitz]

This is a good start. Two things we'd need to proceed with this are:

1. Include the gem as a "development_dependency" in the gemspec, and
2. Include a new .rubocop.yml file, even if it's mostly empty.

These are needed to ensure consistency, so that everyone on the project uses the same version of the gem, and the same rubocop rules.