Support Rack 3 (#607)

* Replace `Rack::File` with `Rack::Files`

`Rack::Files` exists since Rack 2.1. Depend on rack explicitly

ember-cli-rails.gemspec

@@ -17,6 +17,7 @@ Gem::Specification.new do |spec|
 
   spec.add_dependency "ember-cli-rails-assets", ">= 0.6.2", "< 1.0"
   spec.add_dependency "railties", ">= 4.2"
+  spec.add_dependency "rack", ">= 2.1", "< 4.0"
   spec.add_dependency "terrapin", "~> 0.6.0"
   spec.add_dependency "html_page", "~> 0.1.0"
 

lib/ember_cli/deploy/file.rb

@@ -13,7 +13,7 @@ def mountable?
       end
 
       def to_rack
-        Rack::File.new(app.dist_path.to_s, rack_headers)
+        Rack::Files.new(app.dist_path.to_s, rack_headers)
       end
 
       def index_html

spec/lib/ember_cli/deploy/file_spec.rb

@@ -33,7 +33,7 @@
   end
 
   describe "#to_rack" do
-    it "creates a Rack::File instance" do
+    it "creates a Rack::Files instance" do
       deploy = EmberCli::Deploy::File.new(build_app)
 
       rack_app = deploy.to_rack