Reduce the 'build-and-publish- pipleine scope to the master branch

This closes #117, as the
pipeline is skipped since it does not match any event

.drone.yml

@@ -8,6 +8,8 @@ platform:
   arch: amd64
 
 trigger:
+  branch:
+  - master
   event:
     exclude:
     - pull_request
@@ -59,4 +61,3 @@ steps:
   image: cs3org/cs3apis:latest
   commands:
     - cs3apis-build -only-build # compile, build and publish all available languages
-