Upgrade to Ruby 2.6

[jasl]

Have discussed at #669 (comment)

This PR includes

• Ruby minimum 2.6.0
  • Update CI
    • Remove tests against Ruby 2.3 - 2.5
    • Linux OS image upgrade to 20.04
• Bundler bump (CI & Dev only)
• Rubocop bump (dev only)
• ActiveSupport version specification bumped to '>= 5.0', '< 7'
• Upgrade WebMock

In future:

• Upgrade other dependencies to ensure Ruby 3.0+ compatibility
  • I heard macOS will remove Ruby
  • Algolia 2.0 released https://github.com/algolia/algoliasearch-client-ruby/blob/master/upgrade_guide.md
• Enable frozen literal
• CocoaPods files' organize doesn't follow Zeitwerk practice, we can do some refactor then switch to Zeitwerk.
• CocoaPods code styling can be improved by enable new Rubocop lints

[jasl]

@igor-makarov Please review

[igor-makarov]

Can you explain what Zeitwerk is and why it's useful? Also, how hard is it to integrate? Does it have performance benefits?

[jasl]

About the last commit, rubocop outputs

Inspecting 42 files
.........................................W

Offenses:

lib/cocoapods-core/yaml_helper.rb:215:7: W: [Correctable] Lint/UselessAccessModifier: Useless private access modifier.
      private
      ^^^^^^^
lib/cocoapods-core/yaml_helper.rb:294:26: C: [Correctable] Style/TrailingCommaInArguments: Avoid comma after the last parameter of a method call, unless each item is on its own line.
        /\.(nan|NaN|NAN)/, # NaN
                         ^
lib/cocoapods-core/yaml_helper.rb:299:25: C: [Correctable] Style/MutableConstant: Freeze mutable objects assigned to constants.
      INDICATOR_START = /\A#{Regexp.union(INDICATOR_START_CHARS)}/
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
lib/cocoapods-core/yaml_helper.rb:302:31: C: [Correctable] Style/MutableConstant: Freeze mutable objects assigned to constants.
      RESOLVED_TAGS_PATTERN = /\A#{Regexp.union(RESOLVED_TAGS)}\z/
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
lib/cocoapods-core/yaml_helper.rb:305:35: C: [Correctable] Style/MutableConstant: Freeze mutable objects assigned to constants.
      VALID_PLAIN_SCALAR_STRING = %r{\A ...
                                  ^^^^^

42 files inspected, 5 offenses detected, 5 offenses auto-correctable

[jasl]

Can you explain what Zeitwerk is and why it's useful? Also, how hard is it to integrate? Does it have performance benefits?

It's Ruby thread-safe file loader (although CocoaPods no need to care about of thread-safe),
it starts with a very good practice naming convention, then you no longer need do reqiure files in your project.

No performance benefits.

off-topic: For CLI application, maybe https://github.com/Shopify/bootsnap can help to reduce start up time

[jasl]

In addition I upgrade WebMock

[igor-makarov]

@jasl could you rebase on master, discarding changes I pushed - #673 is better and it's already merged.

[jasl]

@jasl could you rebase on master, discarding changes I pushed - #673 is better and it's already merged.

Done

[igor-makarov]

Could you drop the af1c and 5631 commits?

[jasl]

Could you drop the af1c and 5631 commits?

Done

[igor-makarov]

@jasl I've noticed that RuboCop wasn't linting specs anymore so I fixed, see also CocoaPods/shared#11