Travis fix for node 4.

Author: andrewplummer

package.json

@@ -46,6 +46,6 @@
     "rimraf": "^2.4.4",
     "sinon": "^1.15.4",
     "through2": "^2.0.0",
-    "yargs": "^3.15.0"
+    "yargs": "~3.15.0"
   }
 }