Default branch linter and MRI versions out of synch with Blacklight 7.x

[barmintor]

A 7.0.x release should have in gemspec:

  s.add_development_dependency 'rubocop', '~> 0.60.0'
  s.add_development_dependency "rubocop-rspec", '~> 1.8'

and in linter config:

AllCops:
  DisplayCopNames: true
  TargetRubyVersion: 2.3

following Blacklight 7.0.0

[barmintor]

Moved 7.15.x notes to #39

[barmintor]

A thing to note is that Blacklight 7.0.0 is inconsistent in documented MRI expectations:

• README@7.0.0 says 2.2+
• gemspec@7.0.0 says '~> 2.1'
• .rubocop.yml@7.0.0 says TargetRubyVersion: 2.3

I think the right thing for a plugin to do is to align its linter rules with the core blacklight gemspec, since that's what really matters. So I'm going to target 2.1 in this PR.

[barmintor]

Well, that's rude:

• The rubocop dropped support for MRI 2.1 targets after 0.58
• version 0.60.0 won't run with MRI 2.6.x due to a YAML update; needs 0.64.0
• 0.64.0 rubocop will only target MRI 2.2+, so the Blacklight README wins for 7.0.0