update docker to use python 3.7

[ixje]

What current issue(s) does this address, or what feature is it adding?
docker uses python 3.6, whereas the latest version will only work with 3.7

How did you solve this problem?
install 3.7
How did you make sure your solution works?
ran

docker build --no-cache -f Dockerfile -t neopython .

and passed
Are there any special changes in the code that we should be aware of?

Please check the following, if applicable:

• Did you add any tests?
• Did you run make lint?
• Did you run make test?
• Are you making a PR to a feature branch or development rather than master?
• Did you add an entry to CHANGELOG.rst? (if not, please do)

[coveralls]

Coverage remained the same at 85.307% when pulling a625cf0 on ixje:update-docker into 88e0c89 on CityOfZion:development.