Try to workaround the circular dependency on zope.security during tes…

…ting. See https://travis-ci.org/zopefoundation/zope.proxy/jobs/224707855

.travis.yml

@@ -11,6 +11,7 @@ install:
     - pip install -U pip
     - pip install -U setuptools
     - pip install -U zope.testrunner
+    - pip install -U zope.security # We have a circular dependency for testing
     - pip install -e .[test]
 script:
     - zope-testrunner --test-path=src