ci(centos): switch centos away from tiamat builds too

rvm couldn't be found with tiamat images.

.travis.yml

@@ -88,15 +88,15 @@ jobs:
     # - env: INSTANCE=default-ubuntu-2004-tiamat-py3
     # - env: INSTANCE=default-ubuntu-1804-tiamat-py3
     # - env: INSTANCE=default-ubuntu-1604-tiamat-py3
-    - env: INSTANCE=default-centos-8-tiamat-py3
-    - env: INSTANCE=default-centos-7-tiamat-py3
+    # - env: INSTANCE=default-centos-8-tiamat-py3
+    # - env: INSTANCE=default-centos-7-tiamat-py3
     # - env: INSTANCE=default-amazonlinux-2-tiamat-py3
     # - env: INSTANCE=default-oraclelinux-8-tiamat-py3
     # - env: INSTANCE=default-oraclelinux-7-tiamat-py3
     - env: INSTANCE=default-debian-10-master-py3
     - env: INSTANCE=default-ubuntu-2004-master-py3
     - env: INSTANCE=default-ubuntu-1804-master-py3
-    # - env: INSTANCE=default-centos-8-master-py3
+    - env: INSTANCE=default-centos-8-master-py3
     # - env: INSTANCE=default-fedora-32-master-py3
     # - env: INSTANCE=default-fedora-31-master-py3
     # - env: INSTANCE=default-opensuse-leap-152-master-py3
@@ -106,7 +106,7 @@ jobs:
     # - env: INSTANCE=default-ubuntu-2004-3001-py3
     # - env: INSTANCE=default-ubuntu-1804-3001-py3
     # - env: INSTANCE=default-centos-8-3001-py3
-    # - env: INSTANCE=default-centos-7-3001-py3
+    - env: INSTANCE=default-centos-7-3001-py3
     # - env: INSTANCE=default-fedora-32-3001-py3
     # - env: INSTANCE=default-fedora-31-3001-py3
     # - env: INSTANCE=default-opensuse-leap-152-3001-py3