fix(package): update execa to version 4.0.0

Closes #74

Author: greenkeeper[bot]

package.json

@@ -65,7 +65,7 @@
   },
   "dependencies": {
     "debug": "^4.1.1",
-    "execa": "^2.0.3",
+    "execa": "^4.0.0",
     "mkdirp": "^0.5.1",
     "node-fetch": "^2.6.0",
     "openpgp": "^4.5.5",