Update browserify to 4.2.0, which renames internal require.

This updates browserify to 4.2.x, which brings uses
[derequire](https://github.com/calvinmetcalf/derequire) when using the
`--standalone` option. Using `require` inside
a standalone build usually throws off AMD loaders like Require.js.

The tests continue to pass and the only diff in the build is replacing internal
`require`s with `_dereq_`.