Need at least 1 worker
Remove -i argument to browserify

The {} expansion was incorrect:

`-i js/render/draw{vertices,debug}.js` expands to 
`-i js/render/drawvertices.js js/render/drawdebug.js`, not
`-i js/render/drawvertices.js -i js/render/drawdebug.js`.

Thus drawdebug.js was added as an entry file, not as an ignored file.

Furthermore, browserify's -i option appears not to actually work.
Even with `-i js/render/drawvertices.js -i js/render/drawdebug.js`,
drawdebug.js and drawvertices.js are not replaced with empty stubs.

I still don't understand why the build by Travis wasn't broken in
the same way that local builds were, but I've burnt enough time on
this issue already.

Fixes #679