try travis build matrix

.travis.yml

-sudo: required
-
 language: java
 
 services:
   - docker
 
+env:
+  matrix:
+    - DOCKER_IMAGE=stadlerpeter/existdb
+    - DOCKER_IMAGE=stadlerpeter/existdb:3.3.0
+
 before_install:
-  - docker pull stadlerpeter/existdb
+  - docker pull ${DOCKER_IMAGE}
   - ant xar
   - curl http://exist-db.org/exist/apps/public-repo/public/functx-1.0.xar -o build/functx-1.0.xar
-  - docker create --name exist stadlerpeter/existdb
+  - docker create --name exist ${DOCKER_IMAGE}
   - for i in build/*.xar ; do docker cp $i exist:/opt/exist/autodeploy/ ; done
   - docker start exist
   - i=0 ; while (! docker inspect exist | grep -e Status....healthy) ; do if ((i > 30)) ; then echo ' **** timeout ... aborting **** ' ; exit 1 ; else sleep 5 ; echo waiting for container ... ; i=$((i+1)) ; fi ; done