DOC: cleaned references to pandas v0.15 and v0.16 in docs
Last round of cleanup of references to older pandas versions (here v0.15 and v0.16).

I'm not intending to go further up:  v0.16.2 was released in june 2015, and allowing up to 2 years old references to be seen in the docs themselves is reasonable IMO. (but other may have a different view, so if there's a "demand" I could change the cut-off point, its not so much work).

See also #17420 and #17404.

I also removed a reference to python 3.4  in the ``install.rst``. In  the same document, I also "updated" the test section to show python3.6 (just a (very) minor signal that pandas is python 3 positive).