Travis-CI: use documented syntax for virtualenv + system_site_packages

According to https://docs.travis-ci.com/user/languages/python, the
standard way of setting up that configuration is by using the
"system_site_packages" under "virtualenv".

It looks like the current syntax was a valid fix for a Travis-CI
issue that is now fixed, and it was being used in Avocado to support
the now dropped Python 2.6 version checks.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

.travis.yml

 language: python
 
+virtualenv:
+  system_site_packages: true
+
 python:
-    - "2.7_with_system_site_packages"
+    - "2.7"
 
 branches:
     only: