add rubocop gems, ruby to 2.4.1, ignore byebug

[mcritchlow]

Changes:

• ignore byebug history file
• bump ruby to 2.4.1 for rbenv and circle ci
• add rubocop and rubocop-rspec to Gemfile to facilitate local auditing

@ucsdlib/developers - please review

[lsitu]

👍

[VivianChu]

👍