[CI] deactivate travis

If someone wants to maintain that, feel free to do so. But as gitlab CI
is maintained and passing, and travis has been broken for a long time,
while nobody takes care of it, let's just deactivate it.

.travis.yml

@@ -23,10 +23,8 @@ notifications:
 email:
   - [email protected]
 branches:
-only:
-  - master
-  - debian
-  - devel
+  only:
+    - never
 matrix:
 allow_failures:
   - compiler: