Update the docker image to a custom test suite

This changes the images used for the python 2 and 3 CI builds to custom docker images with all requirements and optional requirements pre-installed. The effect is that the CI builds should be simpler and more transparent. Additionally, a fixed lalsuite version will be used.

All images are hosted here: https://hub.docker.com/u/bilbydev/

Instructions for building images

1. Install docker on your local machine
2. Start a container from the image you want to build from (in this case anaconda3)

$ docker run --name test-suite-py3 -it continuumio/anaconda3:latest bash

3. This will drop you to a terminal in the docker. From here, install all the software you want in the usual way
4. Once complete, exit the container ($ exit)
5. Commit the changes

$ docker commit -m "Bilby test suite image" -a "Gregory Ashton" test-suite-py3 bilbydev/test-suite-py3:latest

6. Login and push the changes

$ docker login
$ docker push bilbydev/test-suite-py3