chore(package): update execa to version 4.0.0

Closes #15

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
   },
   "devDependencies": {
     "async-to-gen": "^1.3.3",
-    "execa": "^0.10.0",
+    "execa": "^4.0.0",
     "fixturez": "^1.1.0",
     "fs-extra": "^5.0.0",
     "husky": "^0.14.3",