use nightly tag to pull core openwhisk images from dockerhub (#47)

diff --git a/travis/setup.sh b/travis/setup.sh
index 82ebf3f..d0e6747 100755
--- a/travis/setup.sh
+++ b/travis/setup.sh
@@ -42,7 +42,7 @@
 
 # Deploy OpenWhisk
 cd $WHISKDIR/ansible
-ANSIBLE_CMD="ansible-playbook -i ${WHISKDIR}/ansible/environments/local -e docker_image_prefix=openwhisk"
+ANSIBLE_CMD="ansible-playbook -i ${WHISKDIR}/ansible/environments/local -e docker_image_prefix=openwhisk  -e docker_image_tag=nightly"
 $ANSIBLE_CMD setup.yml
 $ANSIBLE_CMD prereq.yml
 $ANSIBLE_CMD couchdb.yml