Make pkg-resolve an optional dep (#238)

Author: RyanZim

package.json

@@ -19,7 +19,6 @@
   ],
   "dependencies": {
     "object-assign": "^4.0.1",
-    "pkg-resolve": "^0.1.7",
     "postcss": "^5.0.14",
     "postcss-value-parser": "^3.2.3",
     "promise-each": "^2.2.0",
@@ -33,6 +32,9 @@
     "npmpub": "^3.0.1",
     "postcss-scss": "^0.1.3"
   },
+  "optionalDependencies": {
+    "pkg-resolve": "^0.1.7"
+  },
   "jspm": {
     "name": "postcss-import",
     "main": "index.js",