Update package.json (facebook#397)

Author: A-gambit

package.json

@@ -67,7 +67,7 @@
     "jest": "14.1.0",
     "json-loader": "0.5.4",
     "opn": "4.0.2",
-    "path-exists": "^3.0.0",
+    "path-exists": "3.0.0",
     "postcss-loader": "0.9.1",
     "promise": "7.1.1",
     "recursive-readdir": "2.0.0",