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
Just to make sure I verified that assumption:
```console
root@5006d933f5b6:/usr/src/python# pip install hy
Requirement already satisfied (use --upgrade to upgrade): hy in /opt/hylang/hy
...
```
:smile: