Updating tox to specify Python version for integration tests

This change updates the tox configuration for the integration test
suite, specifying that the integration tests should be run with
Python2.7.

tox.ini

@@ -16,6 +16,7 @@ commands = flake8 kmip/
 [testenv:integration]
 # Note: This requires local or remote access to a KMIP appliance or service
 deps = {[testenv]deps}
+basepython=python2.7
 commands =
     py.test --strict kmip/tests/integration {posargs}