Use the official docker images while building hygdrop

Changes Dockerfile to use the hylang image instead of building hy
itself, also requirements file modified to remove earlier dependancies
of hy & astor

Ping @tianon @paultag @copyninja for review
Also, since the `hylang` image already has Hy installed, can't we use the same `requirements.txt` and let the already-installed Hy satisfy the Hy dep?