update cross-env (aframevr#2543)

Author: Kelly Selden

package.json

@@ -53,7 +53,7 @@
     "chai-shallow-deep-equal": "^1.4.0",
     "chalk": "^1.1.3",
     "codecov": "^1.0.1",
-    "cross-env": "^3.1.3",
+    "cross-env": "^4.0.0",
     "exorcist": "^0.4.0",
     "ghpages": "0.0.8",
     "git-rev": "^0.2.1",