Include 404 page in the Ed gem

This is a good lesson in unpacking and checking the contents of gem
files once in awhile to make sure they contain everything one expects!

ed..gemspec

@@ -11,7 +11,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = "http://minicomp.github.io/ed/"
   spec.license       = "MIT"
 
-  spec.files         = `git ls-files -z`.split("\x0").select { |f| f.match(%r{^(assets|optional|_layouts|_includes|_sass|_texts|index|search|about|credits|documentation|atom|Gemfile|LICENSE|README)}i) }
+  spec.files         = `git ls-files -z`.split("\x0").select { |f| f.match(%r{^(404|assets|optional|_layouts|_includes|_sass|_texts|index|search|about|credits|documentation|atom|Gemfile|LICENSE|README)}i) }
 
   spec.add_runtime_dependency "jekyll", "~> 3.6"