Updated Docs to reflect new environ variables.

The environmental variable ``DISTRO`` is no longer valid. Instead,
please use ``OS_NAME`` and ``OS_VERSION``. For most tasks, if you don't
provide a value for ``OS_VERSION``, then tests which make use of this
variable will default to the value of ``7``.
Merge pull request #53 from omaciel/updste-docs

Updated Docs to reflect new environ variables.