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
Why not do something like:
```Dockerfile
COPY requirements-docker.txt /opt/hygdrop/
RUN pip3 install -r requirements-docker.txt
COPY . /opt/hygdrop
```

That way we get to utilize the Docker build cache a little better.