Update .gitlab-ci.yml to make the CI work

.gitlab-ci.yml

-image: debian:testing
-#service:
-#  - python:latest
+image: ghdl/vunit:gcc-master
 
-build:
-  stage: build
-  before_script:
-    - apt-get update
-    - apt-get install -y git make colormake
-    - apt-get install -y gcc-7 gnat-7 gcc gnat
-    - apt-get install -y llvm-4.0 llvm-4.0-dev llvm-4.0-tools llvm
-    - apt-get install -y clang-4.0 clang
-    - apt-get install -y zlib1g-dev libedit-dev
-    - apt-get install -y python3 python3-pip
-#    - ./tools/GitLab-CI/grc.setup.sh
-    - ./tools/GitLab-CI/ghdl.setup.sh
-    - pip3 install -r ./tools/GitLab-CI/requirements.txt
-  script:
-    - echo "build"
-    - export PATH=$PATH:./ghdl/bin
-    - export VUNIT_SIMULATOR=ghdl
-    - python3 ./testbench/run.py -k
-#  artifacts:
-#    paths:
-#      - mybinary
-
-## run tests using the binary built before
-#test:
-#  stage: test
-#  script:
+run_tests:
+  script: python3 ./testbench/run.py