Fix distDocker failing on building core images (#382)

skip distDocker by using prebuilt image for controller and invoker
diff --git a/tools/travis/test_openwhisk.sh b/tools/travis/test_openwhisk.sh
index 4f472a7..698fdd2 100755
--- a/tools/travis/test_openwhisk.sh
+++ b/tools/travis/test_openwhisk.sh
@@ -81,13 +81,11 @@
 #  Set up the OpenWhisk environment for integration testing
 #
 
-#  Build docker images
 cd $OPENWHISK_HOME
-./gradlew --console=plain distDocker -PdockerImagePrefix=testing
 
 #  Fire up the cluster
 cd $OPENWHISK_HOME/ansible
-ANSIBLE_CMD="ansible-playbook -i environments/local -e docker_image_prefix=testing"
+ANSIBLE_CMD="ansible-playbook -i environments/local -e docker_image_prefix=openwhisk"
 $ANSIBLE_CMD setup.yml
 $ANSIBLE_CMD prereq.yml
 $ANSIBLE_CMD couchdb.yml