Environment markers not checked when installing with something else than a wheel

[xavfernandez]

For instance with this setup.py:
https://github.com/aaugustin/websockets/blob/master/setup.py#L40-L42
with:

extras_require={
':python_version=="3.3"': ['asyncio'],
},

We get:

mkvirtualenv py33 --python=/usr/bin/python3.3
(py33)$ pip install --no-use-wheel websockets
 ... sdist being installed ...
(py33)$ pip freeze
websockets==2.3
(py33)$ pip uninstall websockets
... websockets being uninstalled
(py33)$ pip install websockets
... Wheel being installed ...
(py33)$ pip freeze
asyncio==3.4.1
websockets==2.3