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
Yep.. that is why created `requirements-docker` which doesn't include hy.. the plain `requirements` contains hy if you're doing it the traditional way. WDYT?