Don't include python 3.3* in new specs in Python 3

This lets you create new environments in conda with packages that require
Python 2 without explicitly specifying Python 2. Python 3 is still added if it
is already installed into the environment (and the behavior for the Python 2
version of conda is the same as it has always been).
Merge pull request #600 from conda/nopython2

Don't include python 3.3* in new specs in Python 3