Use latest setuptools on TravisCI

Keep seeing build breaks (https://travis-ci.org/gvlproject/cloudbridge/jobs/288572294#L518) and Cryptography indicates it's due to an outdated pip and/or setuptools: https://cryptography.readthedocs.io/en/latest/faq/#installing-cryptography-fails-with-invalid-environment-marker-python-version-3

.travis.yml

@@ -48,6 +48,7 @@ before_install:
            ;;
       esac
 install:
+    - pip install -U setuptools
     - pip install tox
     - pip install coveralls
     - pip install codecov