Add tailwind.mailer.light.config.js to the gem package (#492)

Fixes #26

Since we weren't including this file in the package it was causing
problems for people who were running with the released gems.

But the file would be available if you'd linked your core repos into the
starter repo.

bullet_train-themes-light/bullet_train-themes-light.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
   # spec.metadata["changelog_uri"] = "TODO: Put your gem's CHANGELOG.md URL here."
 
   spec.files = Dir.chdir(File.expand_path(__dir__)) do
-    Dir["{app,config,db,lib}/**/*", "MIT-LICENSE", "Rakefile", "README.md", "tailwind.light.config.js", ".bt-link"]
+    Dir["{app,config,db,lib}/**/*", "MIT-LICENSE", "Rakefile", "README.md", "tailwind.light.config.js", "tailwind.mailer.light.config.js", ".bt-link"]
   end
 
   spec.add_development_dependency "standard"