ci(travis): disabling CentOS7 tests for now

.travis.yml

@@ -13,7 +13,7 @@ env:
   matrix:
     - INSTANCE: v2019-2-py3-debian-9
     - INSTANCE: v2019-2-py3-ubuntu-1804
-    - INSTANCE: v2019-2-py3-centos-7
+    #- INSTANCE: v2019-2-py3-centos-7
     - INSTANCE: v2019-2-py2-fedora-29
 
     - INSTANCE: v2018-3-py2-debian-8