use newer docker image

Jorge Aparicio · Jorge Aparicio · commit 2a8b2c25a4c2 · 2016-08-23T21:19:18.000-05:00
it comes with ruby pre-installed
diff --git a/ci/script.sh b/ci/script.sh
@@ -42,7 +42,7 @@ run_tests() {
 
 main() {
     if [[ $TRAVIS_OS_NAME == "linux" && ${IN_DOCKER_CONTAINER:-n} == "n" ]]; then
-        local tag=2016-08-13
+        local tag=2016-08-24
 
         docker run \
                --privileged \
