Tweaking drone config to better support PRs

.drone.yml

@@ -24,6 +24,9 @@ steps:
     when:
       branch:
         - master
+      event:
+        exclude:
+        - pull_request
 
   - name: notify
     image: plugins/matrix
@@ -34,3 +37,4 @@ steps:
         from_secret: matrix_username
       password:
         from_secret: matrix_password
+