[ci] Execute sonar and coveralls only for pushes

This avoids spinning up VMs for these jobs as they would not
execute any script anyways. Sonar and coveralls are only
updated for pushes.

.travis.yml

@@ -27,10 +27,12 @@ matrix:
       os: linux
       dist: bionic
       env: BUILD=sonar
+      if: "type = push && repo = pmd/pmd && fork = false"
     - name: "linux - run coveralls"
       os: linux
       dist: bionic
       env: BUILD=coveralls
+      if: "type = push && repo = pmd/pmd && fork = false"
     - name: "macosx - mvn verify"
       os: osx
       env: BUILD=deploy