Pull docker image for the api gateway in advance

Author: style95

tools/travis/runStandaloneTests.sh

@@ -47,6 +47,7 @@ kubectl config set-context --current --namespace=default
 # This is required because it is timed out to pull the image during the test.
 docker pull openwhisk/action-nodejs-v10:nightly
 docker pull openwhisk/dockerskeleton:nightly
+docker pull openwhisk/apigateway:0.11.0
 
 cd $ROOTDIR
 TERM=dumb ./gradlew :core:standalone:build \