package.json: add express 3.x to fix docular deps

Docular specifies `express: *` as a dependency, though it does not work
with express 4.x versions.

By adding `express: 3.x` to our package.json, npm will use our copy
of express and don't install the latest express version in docular
node_modules.

This patch is a workaround for gitsome/docular#102 and can be reverted once that fix is landed.

Close #7.

/cc @ritch just FYI, I don't think there is anything to review here.