Update list of supported versions.

Based on versions in Travis CI:

https://github.com/pytoolz/toolz/blob/21bf00cd9c79a4f17efeb3ce1f6e3e3398121cf5/.travis.yml#L4-L10

Author: dhermes

doc/source/install.rst

@@ -11,4 +11,4 @@ three ways:
 
 1.  Toolz is pure Python
 2.  Toolz relies only on the standard library
-3.  Toolz simultaneously supports Python versions 2.6, 2.7, 3.2, 3.3
+3.  Toolz simultaneously supports Python versions 2.6, 2.7, 3.3, 3.4, 3.5, 3.6, PyPy