Add missing files to gemspec

2.15.0 is broken because we're failing to include the backport patch
which we try to `require` at the top of `rubocop.yml`.

This updates the `gempspec` to include all ruby files under `lib`, as
well as the `README.md`. It also switches to use a wildcard to match the
`rubocop.yml` and `rubocop-cli.yml` files, which will be useful when we
add extension configs.

rubocop-shopify.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |s|
   s.email    = "gems@shopify.com"
   s.homepage = "https://shopify.github.io/ruby-style-guide/"
 
-  s.files = ["rubocop.yml", "rubocop-cli.yml", "LICENSE.md"]
+  s.files = Dir["rubocop*.yml", "lib/**/*", "LICENSE.md", "README.md"]
 
   s.metadata = {
     "source_code_uri" => "https://github.com/Shopify/ruby-style-guide/tree/v#{s.version}",