Update groupBy.js

[jgonggrijp]

Hey @Felichz, thank you for going out of your way to contribute to Underscore.

Unfortunately I have to reject this proposal, for a couple of reasons:

• The change does not solve a problem or add new functionality (modern syntax is nice, but it is never a goal in itself).
• The change is not consistent (you updated only one module, while similar changes could apply to most other modules).
• The change introduces a dependency on new syntax that will break the library for many existing users.

Successful pull requests generally include the following elements:

• a very descriptive title;
• a short explanation of the goal of the PR;
• a short explanation of the overall approach;
• changes to both logic and tests if fixing a problem or adding functionality;
• changes to documentation if adding functionality or improving documentation;
• a small increase in code size for a large increase in functionality, if adding functionality;
• no changes that would break the library for existing users.

If you have any questions or comments, please feel free to reply.