Travis-CI: also include 52lts branch

The 52lts branch is missing in the configuration file.  Let's add it.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

.travis.yml

@@ -10,6 +10,7 @@ branches:
     only:
         - master
         - 36lts
+        - 52lts
 
 cache:
     directories: