Show pages from apertis/v2020pre

Use `apertis/v2020pre` branch as default.
Do not test outdated branches.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

.gitlab-ci.yml

-image: docker-registry.apertis.org/apertis/apertis-19.03-testcases-builder
+image: docker-registry.apertis.org/apertis/apertis-v2020pre-testcases-builder
 
 test-renderer:
   stage: test
@@ -14,6 +14,8 @@ test:
       - test
   except:
     - apertis/v2019pre
+    - apertis/v2019dev0
+    - apertis/v2020dev0
 
 pages:
   stage: deploy
@@ -23,4 +25,4 @@ pages:
     paths:
       - public
   only:
-    - apertis/v2020dev0
+    - apertis/v2020pre