travis-ci: enable parallel builds on stretch and sid based images

(cherry picked from commit 9a262f816261fdaf371fbfd34b6460b6221df9ef)

.travis.yml

@@ -3,11 +3,14 @@ language: c
 compiler:
   - gcc
   - clang
+env:
+  - DIST=stretch
+  - DIST=sid
 services:
   - docker
 before_install:
-  - docker pull kamailio/pkg-kamailio-docker:5.1-stretch
-script: docker run -v $TRAVIS_BUILD_DIR:/code:rw kamailio/pkg-kamailio-docker:5.1-stretch /bin/bash -c "export CC=$CC; cd /code; ./test/travis/build_travis.sh"
+  - docker pull kamailio/pkg-kamailio-docker:5.1-$DIST
+script: docker run -v $TRAVIS_BUILD_DIR:/code:rw kamailio/pkg-kamailio-docker:5.1-$DIST /bin/bash -c "cd /code; CC=$CC DIST=$DIST ./test/travis/build_travis.sh"
 branches:
   only:
     - '/^5\.[0-9]+$/'