chore(package): update execa to version 0.8.0

Author: greenkeeper[bot]

package.json

@@ -27,7 +27,7 @@
     "@types/should": "8.3.0",
     "codecov": "2.3.0",
     "dateformat": "2.0.0",
-    "execa": "0.7.0",
+    "execa": "0.8.0",
     "fetch-mock": "5.12.1",
     "http-server": "0.10.0",
     "istanbul-instrumenter-loader": "3.0.0",