Merging pull request 2717

Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

* https://github.com/avocado-framework/avocado:
  Travis-CI: add jobs for nightly versions of CPython
  Travis-CI: add Python 3.5 and 3.6 jobs
  Travis-CI: remove "system_site_packages" option

.travis.yml

 language: python
 
-virtualenv:
-  system_site_packages: true
+matrix:
+  allow_failures:
+    - python: "nightly"
 
 python:
     - "2.7"
     - "3.4"
+    - "3.5"
+    - "3.6"
+    - "nightly"
 
 branches:
     only: