Introduce Rubocop for linting

[aitbw]

In an effort to improve tooling and to achieve a more consistent development environment, this PR aims to include Rubocop to this gem, as well as fix all the offenses based on the Rubocop config file that Solidus uses.

TO-DO:

• [x] Fix Lint/Syntax: unexpected token error raised by this line

[aitbw]

~There's one error present in Travis (locally too!) for Solidus v2.5 —apparently, something is interfering with Bundler when it tries to resolve the dependencies.~

Fixed with #149, will rebase once that PR gets merged.

[tvdeyen]

@aitbw builds should work know. Would you mind to rebase as this is great work I would like to merge?

[aitbw]

Done @tvdeyen :ok_hand:

[tvdeyen]

Thanks