travis: Upgrade to the lastest pytest.

The default pytest included in the travis 2.7 package (3.3.0) is
too old for the pytest_cov package we install. Use pip to upgrade
it before running tests.

Resolves the build failure with python 2.7 on linux.

.travis.yml

@@ -62,7 +62,7 @@ install:
         $PIP install setuptools
         $PIP install ply pep8 mako
         if [ "$MYPYTHON" != "jython" ]; then
-          $PIP install pytest pytest-cov codecov
+          $PIP install --upgrade pytest pytest-cov codecov
         fi
 
 script: