Upgrade to official releases for jupyterhub and configurable-http-proxy

[stefanoborini]

Currently we depend on unreleased versions for both jupyterhub (0.7.0dev) and configurable-http-proxy (1.4.0-dev).

We need to upgrade these dependencies against the official releases when they are ready.

[stefanoborini]

Postponing as there's still no release on this.

[stefanoborini]

Postponing after 0.8.0

[stefanoborini]

Currently upgrading to configurable-http-proxy 2.0.1, which fixes a number of bugs.
There's still no post-0.7 release of jupyterhub, and the current patchlevel is still too old (0.7.2, January), so we prefer to stick to master for now.