fix(package): update execa to version 3.1.0

Closes #50

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
   },
   "dependencies": {
     "caporal": "^0.7.0",
-    "execa": "^0.9.0",
+    "execa": "^3.1.0",
     "has-yarn": "^1.0.0",
     "listr": "^0.12.0",
     "rxjs": "^5.4.0",