tests: stop burning CPU until this one job is fixed.

.travis.yml

@@ -10,11 +10,11 @@ python:
 - "2.7"
 
 env:
-- MODE=mitogen MITOGEN_TEST_DISTRO=debian
+#- MODE=mitogen MITOGEN_TEST_DISTRO=debian
-- MODE=mitogen MITOGEN_TEST_DISTRO=centos
+#- MODE=mitogen MITOGEN_TEST_DISTRO=centos
-- MODE=debops
+#- MODE=debops
 - MODE=ansible ANSIBLE_VERSION=2.4.3.0 MITOGEN_TEST_DISTRO=debian
-- MODE=ansible ANSIBLE_VERSION=2.5.0 MITOGEN_TEST_DISTRO=centos
+#- MODE=ansible ANSIBLE_VERSION=2.5.0 MITOGEN_TEST_DISTRO=centos
 
 install:
 - pip install -r dev_requirements.txt