Update to express 4

Middlewares are no longer included by default in express. Had to make some configuration changes to server.js

The error was happening because in package.json the dependency was set to `*`. From now we have moved into a more **stable** 4.0.x dependency version of express. With this we hope to not break any more builds :) 

We should probably apply all these dependency conservativeness to all our other deps in package.json

Fixes issue #15 - Thanks to @plediii