Avoid func declarations using function keyword (#2813)

We had only a few function expressions declared with the function keyword; almost everything is using function declarations.

This came up after this discussion: https://github.com/badges/shields/pull/2803#discussion_r249011621 though is actually not related ot that example.

What’s being removed is a third option, which is assigning to a variable a function expression using the `function` keyword. There’s still room for the programmer to choose between arrow function expressions and function declarations.

services/response-fixtures.js

@@ -1,9 +1,9 @@
 'use strict'
 
-const invalidJSON = function() {
-  return [200, '{{{{{invalid json}}', { 'Content-Type': 'application/json' }]
-}
-
 module.exports = {
-  invalidJSON,
+  invalidJSON: () => [
+    200,
+    '{{{{{invalid json}}',
+    { 'Content-Type': 'application/json' },
+  ],
 }