client: Change default from ES5 to ES6

This change matches that for MediaWiki 1.41. Code that explicitly wishes
to support ES5 clients still can extend wikimedia/client-es5, as has
been recommended for a while.

Ref https://phabricator.wikimedia.org/T178356.
Closes #492.

client.json

@@ -1,5 +1,5 @@
 {
 	"extends": [
-		"./client-es5"
+		"./client-es6"
 	]
 }