chore(package): update execa to version 0.9.0

Author: greenkeeper[bot]

package.json

@@ -62,7 +62,7 @@
     "autoprefixer": "7.2.4",
     "ava": "0.23.0",
     "coveralls": "2.13.2",
-    "execa": "0.8.0",
+    "execa": "0.9.0",
     "nyc": "11.4.1",
     "standard-version": "4.3.0",
     "text-loader": "0.0.1",