Unpin prettier version to allow patch upgrades

Closes postcss#219

Author: RyanZim

package.json

@@ -38,7 +38,7 @@
     "eslint-config-problems": "1.0.0",
     "nyc": "^11.0.2",
     "postcss-import": "^11.0.0",
-    "prettier": "1.13.0",
+    "prettier": "~1.13.0",
     "sugarss": "^1.0.0",
     "uuid": "^3.0.1"
   },