👷 : update sonar config

fetching master ref to help sonar collecting changed files in the
branch.
it resolves this warning :
[INFO] SCM collecting changed files in the branch
[WARNING] Could not find ref: master in refs/heads or refs/remotes/origin
[INFO] SCM collecting changed files in the branch (done) | time=73ms

.travis.yml

@@ -10,6 +10,8 @@ addons:
       secure: 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
 
 script:
+  # fetching master refs when building other branches helps sonar computing PRs
+  - git fetch origin +refs/heads/master:refs/remotes/origin/master
   # the following command line builds the project, runs the tests with coverage and then execute the SonarCloud analysis
-  - mvn clean org.jacoco:jacoco-maven-plugin:prepare-agent install sonar:sonar
+  - mvn clean org.jacoco:jacoco-maven-plugin:prepare-agent verify sonar:sonar