fix numpy version to 1.16 for CI

Author: JasperGeurtz

Diff for: .travis.yml

@@ -13,6 +13,7 @@ before_script:
   - git clone https://github.com/Bytekeeper/sc-docker.git
   - cp it/sc-docker-support/*.dockerfile sc-docker/docker/dockerfiles
   - pushd sc-docker
+  - pip3 install numpy==1.16.6
   - python3 setup.py bdist_wheel
   - pip3 install dist/scbw*.whl
   - '[ ! -f /tmp/sc-docker/starcraft.zip ] || (cp /tmp/sc-docker/starcraft.zip scbw/local_docker && echo "Using cached starcraft.zip")'