Add JavaScript rule to return a value in callbacks

It's very easy for Ruby developers to omit the `return` statement in an
array callback, such as the ones in `map` or `filter`. Doing so will
make those funcions have the same effect as `forEach`, which is usually
not the intended behaviour.

.eslintrc.yml

@@ -14,6 +14,7 @@ parserOptions:
   ecmaVersion: 5
 rules:
   array-bracket-spacing: error
+  array-callback-return: error
   block-spacing: error
   brace-style: error
   comma-spacing: error