elm: use resource patch

Formula/elm.rb

@@ -13,6 +13,13 @@ class Elm < Formula
     resource "elm-package" do
       url "https://github.com/elm-lang/elm-package/archive/0.18.0.tar.gz"
       sha256 "5cf6e1ae0a645b426c0474cc7cd3f7d1605ffa1ac5756a39a8b2268ddc7ea0e9"
+
+      # Fix two "Not in scope" errors
+      # Upstream PR from 3 Oct 2017 "Paths.hs: fix build failure"
+      patch do
+        url "https://github.com/elm-lang/elm-package/pull/287.patch?full_index=1"
+        sha256 "3f922d7962a41217e760361ad444d00676aff0e40c3741fd536b39d2961165d3"
+      end
     end
 
     resource "elm-make" do
@@ -41,13 +48,6 @@ class Elm < Formula
   depends_on "ghc@8.0" => :build
   depends_on "cabal-install" => :build
 
-  # Fix two "Not in scope" errors
-  # Upstream PR from 3 Oct 2017 "Paths.hs: fix build failure"
-  resource "elm-package-patch" do
-    url "https://github.com/elm-lang/elm-package/pull/287.patch?full_index=1"
-    sha256 "3f922d7962a41217e760361ad444d00676aff0e40c3741fd536b39d2961165d3"
-  end
-
   def install
     # elm-compiler needs to be staged in a subdirectory for the build process to succeed
     (buildpath/"elm-compiler").install Dir["*"]
@@ -70,11 +70,6 @@ class Elm < Formula
               "HTTP >= 4000.2.5 && < 4000.4," # 4000.3.3 is current
     end
 
-    resource("elm-package-patch").stage do
-      system "patch", "-p1", "-i", Pathname.pwd/"287.patch", "-d",
-             buildpath/"elm-package"
-    end
-
     cabal_sandbox do
       cabal_sandbox_add_source "elm-compiler", *extras
       cabal_install "--only-dependencies", "elm-compiler", *extras