Pin version of docker client on travis

Newer versions of docker-py use a default API version of 1.39 which is too new for the version of docker on travis.

Author: betatim

.travis.yml

@@ -17,6 +17,7 @@ install:
   - npm install
   - npm run webpack
   - pip install --upgrade . -r helm-chart/images/binderhub/requirements.txt
+  - pip install -U "docker~=4.2.0"
   - pip freeze
 
 script: