ci(travis): fix CentOS instance name

.travis.yml

@@ -39,7 +39,7 @@ env:
     # - INSTANCE: default-amazonlinux-2-2018-3-py2
     # - INSTANCE: default-debian-8-2017-7-py2
     # - INSTANCE: default-ubuntu-1604-2017-7-py2
-    - INSTANCE: centos6-centos-6-2017-7-py2
+    - INSTANCE: default-centos-6-2017-7-py2
     # - INSTANCE: default-fedora-29-2017-7-py2
     # - INSTANCE: default-opensuse-leap-15-2017-7-py2
     # - INSTANCE: default-amazonlinux-2-2017-7-py2