chore(package): update execa to version 2.0.4

Closes #13

Author: greenkeeper[bot]

package.json

@@ -57,7 +57,7 @@
   "devDependencies": {
     "ava": "1.3.0",
     "coveralls": "3.0.3",
-    "execa": "1.0.0",
+    "execa": "2.0.4",
     "nyc": "13.3.0"
   }
 }