Manually define travis matrix.
Manually define travis matrix.

Travis-CI no longer has all supported Python versions installed by default.
Each version needs to be set via the `python` setting. However, we don't
want a matrix where every Python version runs with every `TOXENV`.
Therefore, we now manually define the matrix so that the Python version
is properly matched with the TOXENV.

waylan/Python-Markdown,waylan/Python-Markdown