update execa to 4.x

BREAKING CHANGE: Requeires Node 10.x

Author: pooya parsa

package.json

@@ -16,7 +16,7 @@
   },
   "dependencies": {
     "defu": "^1.0.0",
-    "execa": "^3.0.0",
+    "execa": "^4.0.0",
     "fs-extra": "^8.1.0",
     "hasha": "^5.1.0",
     "jimp-compact": "^0.9.3",