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
No, I mean to say that you don't need to split them, do you?  Isn't `pip` smart enough to notice that the `hy>=0.10` dep is already satisfied and skip it appropriately?