feat: formatter.js es6 (#87)

Signed-off-by: Grant Timmerman <timmerman+devrel@google.com>

lib/formats/json/formatter.js

@@ -1,18 +1,15 @@
+class JSONFormatter {
+  /*
+   * Every internal data structure is JSON by nature, so
+   * no transformation is required
+   */
+  format(payload) {
+    return payload;
+  }
 
-function JSONFormatter() {
-
+  toString(payload) {
+    return JSON.stringify(payload);
+  }
 }
 
-/*
- * Every internal data structure is JSON by nature, so
- * no transformation is required
- */
-JSONFormatter.prototype.format = function(payload) {
-  return payload;
-};
-
-JSONFormatter.prototype.toString = function(payload) {
-  return JSON.stringify(payload);
-};
-
 module.exports = JSONFormatter;