Jekyll: Update dependency constraint to allow for rubocop-ast v0.7.1

Hi, I've updated this gem version in Gemfle, ran the tests, and all the tests have passed

/script/test 
+ ruby -S bundle exec rake TESTOPTS=--profile test
/home/hyde/.rvm/rubies/ruby-2.6.3/bin/ruby -w -I"lib:lib:test" -I"/home/hyde/.rvm/gems/ruby-2.6.3/gems/rake-13.0.1/lib" "/home/hyde/.rvm/gems/ruby-2.6.3/gems/rake-13.0.1/lib/rake/rake_test_loader.rb" "test/test_ansi.rb" "test/test_cleaner.rb" "test/test_coffeescript.rb" "test/test_collections.rb" "test/test_command.rb" "test/test_commands_serve.rb" "test/test_commands_serve_servlet.rb" "test/test_configuration.rb" "test/test_convertible.rb" "test/test_data_reader.rb" "test/test_doctor_command.rb" "test/test_document.rb" "test/test_drop.rb" "test/test_entry_filter.rb" "test/test_excerpt.rb" "test/test_excerpt_drop.rb" "test/test_filters.rb" "test/test_front_matter_defaults.rb" "test/test_generated_site.rb" "test/test_kramdown.rb" "test/test_layout_reader.rb" "test/test_liquid_extensions.rb" "test/test_liquid_renderer.rb" "test/test_log_adapter.rb" "test/test_new_command.rb" "test/test_page.rb" "test/test_page_without_a_file.rb" "test/test_path_sanitization.rb" "test/test_plugin_manager.rb" "test/test_post_reader.rb" "test/test_regenerator.rb" "test/test_related_posts.rb" "test/test_sass.rb" "test/test_site.rb" "test/test_site_drop.rb" "test/test_static_file.rb" "test/test_tags.rb" "test/test_theme.rb" "test/test_theme_assets_reader.rb" "test/test_url.rb" "test/test_utils.rb" --profile
# -------------------------------------------------------------
# SPECS AND TESTS ARE RUNNING WITH WARNINGS OFF.
# SEE: https://github.com/Shopify/liquid/issues/730
# SEE: https://github.com/jekyll/jekyll/issues/4719
# -------------------------------------------------------------

# Running tests with run options --profile --seed 907:

.........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Finished tests in 112.201639s, 8.0658 tests/s, 14.3848 assertions/s.

I also ran cucumber, and all is passing, you can see the preview here: https://reports.cucumber.io/reports/1ec622a5-8964-4b58-8da7-3078c7e4a3f0

Anyway, this #8418 mentioned that we should not update the gem. So, I ran the suite to ensure that anything break. Can I do a PR to update the gem version? Or should we close this PR? @ashmaroli

Our bot is not aware that we are limiting that dependency for a particular platform. Ignore it.

Resolved via https://github.com/jekyll/jekyll/commit/7d8a839a2132cadd940a3b4f8d7c5f9f6b0f9f62