Move native modules to optionalDependencies

Fixes #3016.

package.json

@@ -4,7 +4,9 @@
     "asar": "^0.8.0",
     "coffee-script": "^1.9.2",
     "coffeelint": "^1.9.4",
-    "request": "*",
+    "request": "*"
+  },
+  "optionalDependencies": {
     "runas": "^3.0.0"
   },
   "private": true,

spec/package.json

@@ -5,14 +5,16 @@
   "version": "0.1.0",
   "devDependencies": {
     "basic-auth": "^1.0.0",
-    "ffi": "2.0.0",
     "formidable": "1.0.16",
     "graceful-fs": "3.0.5",
     "mocha": "2.1.0",
     "q": "0.9.7",
-    "runas": "3.x",
     "temp": "0.8.1",
     "walkdir": "0.0.7",
     "ws": "0.7.2"
+  },
+  "optionalDependencies": {
+    "ffi": "2.0.0",
+    "runas": "3.x"
   }
 }