Update .gitlab-ci.yml

.gitlab-ci.yml

 stages:
   - build
 
-build_python2.7:
-  stage: build
-  image: docker:18.09.7
-  only:
-    - master
-  services:
-    - docker:18.09.7-dind
-  before_script:
-    - ''
-  script:
-    - docker build -t clarinpl/python:2.7 2.7/
-    - echo $DOCKER_PASSWORD > pass.txt
-    - cat pass.txt | docker login --username $DOCKER_USERNAME --password-stdin
-    - rm pass.txt
-    - docker push clarinpl/python:2.7
-    
 build_python3.6:
   stage: build
   image: docker:18.09.7