Fix Tests

[jormon]

Rubocop added a new default cop and renamed another.

Also, second commit fixes the fact that haml_lint dropped support for ruby < 2.4 so install an older package for that.

[Arcanemagus]

It looks like Ruby 2.3.x is no longer supported by Ruby in the first place, would you mind updating this to drop testing for that and include Ruby 2.6 and 2.7? We should probably drop Ruby 2.4 as well since it will EoL soon.

[jormon]

@Arcanemagus i tried making the update but the tests are failing on travis. I cannot get the tests to run locally (never done atom development before), they all fail like this:

linter-haml fix-tests % atom --test ./spec/linter-haml-spec.js
FFF

The haml-lint provider for Linter
  it checks a file with issues
    Error: Failed to load package 'language-haml'
      at PackageManager.activatePackage (/Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:798:29)
      at /Users/jormon/playground/linter-haml/spec/linter-haml-spec.js:22:25
      at Generator.next (<anonymous>)
      at step (/Users/jormon/playground/linter-haml/spec/linter-haml-spec.js:3:273)
    Expected 3 not to be greater than 2.
      Error: Expected 3 not to be greater than 2.
      at /Users/jormon/playground/linter-haml/spec/linter-haml-spec.js:45:33
      at Generator.next (<anonymous>)
      at step (/Users/jormon/playground/linter-haml/spec/linter-haml-spec.js:3:273)
      at process._tickCallback (internal/process/next_tick.js:68:7)
  it finds nothing wrong with a valid file
    Error: Failed to load package 'language-haml'
      at PackageManager.activatePackage (/Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:798:29)
      at /Users/jormon/playground/linter-haml/spec/linter-haml-spec.js:22:25
      at Generator.next (<anonymous>)
      at step (/Users/jormon/playground/linter-haml/spec/linter-haml-spec.js:3:273)
  it finds nothing wrong with an empty file
    Error: Failed to load package 'language-haml'
      at PackageManager.activatePackage (/Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:798:29)
      at /Users/jormon/playground/linter-haml/spec/linter-haml-spec.js:22:25
      at Generator.next (<anonymous>)
      at step (/Users/jormon/playground/linter-haml/spec/linter-haml-spec.js:3:273)


Finished in 2.9 seconds
3 tests, 19 assertions, 4 failures, 0 skipped

Is there a primer on how to get the test running locally? Sorry for my first-time atom hacking questions, but their guide and the README.md don't provide a lot of clues on how to run atom tests. Would be happy to submit a doc diff on that if you can point me in the right direction. Thanks!