Speed up generate.js

`regenerate#toRegExp` calls `regenerate#toString` and then just wraps it in `RegExp`. `generate.js` used to take the result of `toRegExp` and then coerce it back into a string. This small change skips these two redundant steps by just using the result of `toString` directly.
Merge pull request #1 from mathiasbynens/minor-speed-up

Speed up generate.js