This commit decouples the test virtualenv creation from tox and simplify
it for running both locally and on travis-ci. Creation of the test
virtualenvs now happen via Makefile target 'test-env' which will create
2 virtualenvs in the dir tests/virtualenvs and also dump their
respective package data in .pickle files that will be used by the tests.
Also, builds on travis now no longer depend on tox but reuses some of
the Makefile targets for test virtualenv creation/cleanup.
ciarancourtney