travis: build using pkg-kamailio-docker 5.2-$DIST tag

.travis.yml

@@ -9,8 +9,8 @@ env:
 services:
   - docker
 before_install:
-  - docker pull kamailio/pkg-kamailio-docker:dev-$DIST
-script: docker run -v $TRAVIS_BUILD_DIR:/code:rw kamailio/pkg-kamailio-docker:dev-$DIST /bin/bash -c "cd /code; CC=$CC DIST=$DIST ./test/travis/build_travis.sh"
+  - docker pull kamailio/pkg-kamailio-docker:5.2-$DIST
+script: docker run -v $TRAVIS_BUILD_DIR:/code:rw kamailio/pkg-kamailio-docker:5.2-$DIST /bin/bash -c "cd /code; CC=$CC DIST=$DIST ./test/travis/build_travis.sh"
 branches:
   only:
     - 'master'