Fix travis tests

Author: J-M0

.travis.yml

@@ -45,8 +45,8 @@ jobs:
 install:
   - sed -i -e 's/^/#/' tests/gnupghome/gpg-agent.conf
   - ./install_dependencies.${TRAVIS_OS_NAME}.sh
-  # ensure tox and coveralls are installed
-  - pip install tox
+  # ensure tox and virtualenv are up to date
+  - pip install -U tox virtualenv
 
 # set TOXENV if it isn't yet
 before_script: