change travis builds to publish docker images with tag 'nightly' (#27)

diff --git a/.travis.yml b/.travis.yml
index 4b0ae18..73335d2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -36,7 +36,7 @@
     all_branches: true
     repo: apache/incubator-openwhisk-runtime-ruby
 - provider: script
-  script: "./tools/travis/publish.sh openwhisk 2.5 latest && ./tools/travis/publish.sh openwhisk 2.6 latest"
+  script: "./tools/travis/publish.sh openwhisk 2.5 nightly && ./tools/travis/publish.sh openwhisk 2.6 nightly"
   on:
     branch: master
     repo: apache/incubator-openwhisk-runtime-ruby
diff --git a/tools/travis/publish.sh b/tools/travis/publish.sh
index 4d0330b..965d4b9 100755
--- a/tools/travis/publish.sh
+++ b/tools/travis/publish.sh
@@ -47,8 +47,8 @@
 -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 \