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.
Merge pull request #8 from strongloop/feature/fix-doc-on-windows

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