change travis builds to publish docker images with tag 'nightly' (#139)

diff --git a/.travis.yml b/.travis.yml
index e62ba20..4b013af 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -41,7 +41,7 @@
       all_branches: true
       repo: apache/incubator-openwhisk-runtime-nodejs
   - provider: script
-    script: "./tools/travis/publish.sh openwhisk 6 latest && ./tools/travis/publish.sh openwhisk 8 latest && ./tools/travis/publish.sh openwhisk 10 latest && ./tools/travis/publish.sh openwhisk 12 latest"
+    script: "./tools/travis/publish.sh openwhisk 6 nightly && ./tools/travis/publish.sh openwhisk 8 nightly && ./tools/travis/publish.sh openwhisk 10 nightly && ./tools/travis/publish.sh openwhisk 12 nightly"
     on:
       branch: master
       repo: apache/incubator-openwhisk-runtime-nodejs
diff --git a/tools/travis/publish.sh b/tools/travis/publish.sh
index e2ca1c3..6c4807d 100755
--- a/tools/travis/publish.sh
+++ b/tools/travis/publish.sh
@@ -51,8 +51,8 @@
 -PdockerImagePrefix=${IMAGE_PREFIX} \
 -PdockerImageTag=${IMAGE_TAG}
 
-  # if doing latest also push a tag with the hash commit
-  if [ ${IMAGE_TAG} == "latest" ]; then
+  # if doing nightly also push a tag with the hash commit
+  if [ ${IMAGE_TAG} == "nightly" ]; then
   SHORT_COMMIT=`git rev-parse --short HEAD`
   TERM=dumb ./gradlew \
   :core:${RUNTIME}:distDocker \