Build fails because of missing python version

[carlosms]

As can be seen here: https://travis-ci.org/src-d/lookout-sdk/jobs/481415428#L368

It looks like the travis xenial image has changed the default python version from 3.6 to 3.7. I think we can't set a python version for a job that already has language: go; go: '1.10'. So we either install python 3.6, or generate the code with 3.7.