change travis builds to publish docker images with tag 'nightly' (#63)

diff --git a/.travis.yml b/.travis.yml
index 92f0af4..114d45a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -41,17 +41,17 @@
       all_branches: true
       repo: apache/incubator-openwhisk-runtime-php
   - provider: script
-    script: "./tools/travis/publish.sh openwhisk 7.1 latest"
+    script: "./tools/travis/publish.sh openwhisk 7.1 nightly"
     on:
       branch: master
       repo: apache/incubator-openwhisk-runtime-php
   - provider: script
-    script: "./tools/travis/publish.sh openwhisk 7.2 latest"
+    script: "./tools/travis/publish.sh openwhisk 7.2 nightly"
     on:
       branch: master
       repo: apache/incubator-openwhisk-runtime-php
   - provider: script
-    script: "./tools/travis/publish.sh openwhisk 7.3 latest"
+    script: "./tools/travis/publish.sh openwhisk 7.3 nightly"
     on:
       branch: master
       repo: apache/incubator-openwhisk-runtime-php
diff --git a/tools/travis/publish.sh b/tools/travis/publish.sh
index 42e37ef..5b74844 100755
--- a/tools/travis/publish.sh
+++ b/tools/travis/publish.sh
@@ -41,8 +41,8 @@
 -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 [ ${IMAGE_TAG} == "nightly" ]; then
   SHORT_COMMIT=`git rev-parse --short HEAD`
   TERM=dumb ./gradlew \
   :core:php${RUNTIME_VERSION}Action:distDocker \