Change CI to include `make push`

This replaces the makes for each python version with a simple `make push`.
It will also add a "main image" for each version, i.e. `gsiupm/senpy:1.0.0` in
addition to `gsiupm/senpy:1.0.0-python2.7` and `gsiupm/senpy:1.0.0-python3.5`.

.gitlab-ci.yml

@@ -31,29 +31,19 @@ test-2.7:
   variables:
     PYTHON_VERSION: "2.7"
 
-.image: &image_definition
+push:
   stage: push
   script:
-    - make -e push-$PYTHON_VERSION
+    - make -e push
   only:
     - tags
     - triggers
     - fix-makefiles
 
-push-3.5:
-  <<: *image_definition
-  variables:
-    PYTHON_VERSION: "3.5"
-
-push-2.7:
-  <<: *image_definition
-  variables:
-    PYTHON_VERSION: "2.7"
-
 push-latest:
-  <<: *image_definition
-  variables:
-    PYTHON_VERSION: latest
+  stage: push
+  script:
+    - make -e push-latest
   only:
     - master
     - triggers