Enable mergify strict mode

In the past we had strict mode in branch protection. This didn't really work as it broke mergify. Now mergify supports this options and takes care of the merging automatically. Let's see how it goes ;)

Reason is the recent build failures we had on master, during a busy merge day. This could have been prevented if sequential PR's where re-merging with master.

More info: https://doc.mergify.io/strict-workflow.html

.mergify.yml

@@ -6,6 +6,7 @@ pull_request_rules:
     actions:
       merge:
         method: merge
+        strict: true
   - name: Trusted author, successful travis and 1 approved review
     conditions:
       - author~=(kaiyou|muhlemmer|mildred|HorayNarea|adi90x|hoellen|ofthesun9)
@@ -14,3 +15,4 @@ pull_request_rules:
     actions:
       merge:
         method: merge
+        strict: true