Revert "Update diff-dom package to 3.1.0"

This reverts commit 01b97986f6.

package.json

@@ -20,7 +20,7 @@
   "dependencies": {
     "@babel/core": "7.4.0",
     "@babel/preset-env": "7.4.2",
-    "diff-dom": "https://github.com/fiduswriter/diffDOM/archive/v3.1.0.tar.gz",
+    "diff-dom": "2.3.1",
     "govuk-elements-sass": "3.1.2",
     "govuk_frontend_toolkit": "8.1.0",
     "govuk_template_jinja": "0.24.1",
@@ -45,8 +45,5 @@
     "gulp-sass-lint": "1.4.0",
     "jshint": "2.10.2",
     "jshint-stylish": "2.2.1"
-  },
-  "peerDependencies": {
-    "rollup": "1.10.0"
   }
 }