optimized some require calls

According to @shtylman 's [post](https://github.com/substack/node-browserify/issues/296#issuecomment-14097374) I removed some duplicate require calls. I located them with a little tool. I was interested in the difference.

Here are the results:

| bundler | before | after | before (gz) | after (gz) | diff |
|---|---|---|---|---|---|
| browserify | 159,382B | 158,936B | 48,643B | 48,519B | -124B |
| webpack | 141,231B | 141,163B | 45,795B | 45,799B | +5B |