Added missing python-coverage to testing docker image.

.gitlab-ci.yml

@@ -1,5 +1,5 @@
 ---
-image: s3lph/matemat-ci:20180802-01
+image: s3lph/matemat-ci:20180802-02
 
 stages:
 - test

testing/Dockerfile

@@ -9,7 +9,7 @@ RUN sed -re 's/stretch/buster/g' -i /etc/apt/sources.list \
   && chown matemat:matemat -R /var/matemat/upload \
   && apt-get update -qy \
   && apt-get install -y --no-install-recommends file sudo openssh-client git docker.io build-essential \
-  && python3.7 -m pip install wheel pycodestyle mypy \
+  && python3.7 -m pip install coverage wheel pycodestyle mypy \
   && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /home/matemat