Fix version string (#669)

Author: fhinkel

.kokoro/build-with-appengine.sh

@@ -27,7 +27,7 @@ export NODE_ENV=development
 
 # Strip appengine and flexible/standard from version string.
 VERSION=$(echo $PROJECT | sed 's_appengine/__')
-VERSION=$(echo $VERSION | sed 's_/flexible\|standard__')
+VERSION=$(echo $VERSION | sed 's_/flexible\|/standard__')
 
 export GAE_VERSION=$VERSION