Use string types from the proper version of python

dumps makes several type comparisons in one of its generation steps. One
of these is against the string type. But the string type used is
specific to Python 2, and it fails on Python 3.

This commit uses six ( https://pythonhosted.org/six/ ) to compare to
the proper string type according to the Python version being used.
Add six to setup dependencies
Add instalation of six to travis' before_install
Merge pull request #6 from jparvela/python3dumps

Use the proper string type according to python version