Use maintainable dependencies for testing

[rominf]

I'm Fedora maintainer and I'm in the process of packaging python-ovh for Fedora. Unfortunately, python-ovh relies on yanc (not in the repos, newest version released: Dec 16, 2014) and old nose (Fedora has newer, incompatible version), which makes it hard to test the library. Please consider updating testing dependencies (tox/pytest would be perfect).