Modernize dependencies, code style, etc.

[Mange]

Modernize all the things. This repo has been stale for some time now, so it's time to install some upgrades.

• Dropped Ruby 2.2, 2.3, 2.4.
• Added support for Ruby 2.6.
• Dropped Rails 4.x and 5.0.
• Drop Guard gem for development.
• Raise minimum versions of some gems.
• Add rubocop config and fix all style violations.

[codecov-io]

Codecov Report

Merging #87 into master will increase coverage by 1.24%. The diff coverage is 99.46%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage    98.4%   99.64%   +1.24%     
==========================================
  Files          16       18       +2     
  Lines         502      566      +64     
==========================================
+ Hits          494      564      +70     
+ Misses          8        2       -6

Impacted Files	Coverage Δ
lib/roadie/rails/document_builder.rb	100% <ø> (ø)	:arrow_up:
lib/roadie/rails/inline_on_delivery.rb	100% <ø> (+42.85%)	:arrow_up:
lib/roadie/rails/automatic.rb	100% <ø> (ø)	:arrow_up:
lib/roadie/rails/mailer.rb	100% <ø> (ø)	:arrow_up:
spec/lib/roadie/rails/mail_inliner_spec.rb	100% <100%> (ø)	:arrow_up:
lib/roadie/rails/options.rb	100% <100%> (ø)	:arrow_up:
lib/roadie/rails/utils.rb	100% <100%> (ø)
spec/lib/roadie/rails/mailer_spec.rb	100% <100%> (ø)	:arrow_up:
spec/lib/roadie/rails/automatic_spec.rb	100% <100%> (ø)	:arrow_up:
lib/roadie/rails/mail_inliner.rb	100% <100%> (ø)	:arrow_up:
... and 11 more

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