chore(package): update execa to version 3.1.0

Closes #70

Author: greenkeeper[bot]

package.json

@@ -17,7 +17,7 @@
     "eslint-plugin-node": "^4.2.2",
     "eslint-plugin-promise": "^3.5.0",
     "eslint-plugin-standard": "^3.0.1",
-    "execa": "^0.10.0",
+    "execa": "^3.1.0",
     "fixpack": "^2.3.1",
     "nyc": "^11.0.1"
   },