dev: lock rubocop-shopify dependency for now

because

a) we can't generate the ruby 2.6 ci image with the latest version of
   the gem (which dropped support for ruby <2.7).
b) we kept getting CI breaks from eager updating, anyway

At some point, I'd like to move all our development dependencies into
the Gemfile, tighten the dependencies, and have dependabot send PRs
for updates.

Author: flavorjones

nokogiri.gemspec

@@ -349,7 +349,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency("rubocop-minitest", "~> 0.17")
   spec.add_development_dependency("rubocop-performance", "~> 1.12")
   spec.add_development_dependency("rubocop-rake", "~> 0.6")
-  spec.add_development_dependency("rubocop-shopify", "~> 2.3")
+  spec.add_development_dependency("rubocop-shopify", "= 2.5.0") # TODO: loosen this after dropping support for Ruby 2.6
   spec.add_development_dependency("ruby_memcheck", "~> 1.0")
   spec.add_development_dependency("simplecov", "~> 0.21")