roll back to execa 2.0.0 to support Node 8.0.0

Author: jennifer-shehane

package.json

@@ -51,7 +51,7 @@
   "dependencies": {
     "@cypress/browserify-preprocessor": "2.1.4",
     "debug": "4.1.1",
-    "execa": "4.0.0"
+    "execa": "2.0.0"
   },
   "devDependencies": {
     "@babel/core": "7.8.4",