Failed to install in a tox created venv

Hi.

I couldn't cause the package to be installed in a tox environment. A simple reproduction test:

```
[tox]
envlist = py27

[testenv]
commands = python -c 'print "OK"'
deps = pyuv
uzmq
```

Fails

```
Collecting pyuv
Using cached pyuv-1.0.2.tar.gz
Collecting uzmq
Using cached uzmq-0.3.1.tar.gz
Traceback (most recent call last):
File "", line 20, in
File "/var/folders/48/t51nvxxn3l11p8bmss8_sjq40000gn/T/pip-build-vhxPNK/uzmq/setup.py", line 10, in
from uzmq import __version__
File "uzmq/__init__.py", line 16, in
from uzmq.poll import ZMQPoll
File "uzmq/poll.py", line 9, in
import pyuv
ImportError: No module named pyuv
Complete output from command python setup.py egg_info:
Traceback (most recent call last):

File "", line 20, in

File "/var/folders/48/t51nvxxn3l11p8bmss8_sjq40000gn/T/pip-build-vhxPNK/uzmq/setup.py", line 10, in

from uzmq import __version__

File "uzmq/__init__.py", line 16, in

from uzmq.poll import ZMQPoll

File "uzmq/poll.py", line 9, in

import pyuv

ImportError: No module named pyuv

----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /var/folders/48/t51nvxxn3l11p8bmss8_sjq40000gn/T/pip-build-vhxPNK/uzmq

ERROR: could not install deps [pyuv, uzmq]; v = InvocationError('/private/tmp/1/.tox/py27/bin/pip install --pre pyuv uzmq (see /private/tmp/1/.tox/py27/log/py27-1.log)', 1)
_________________________________________________________________ summary __________________________________________________________________
ERROR: py27: could not install deps [pyuv, uzmq]; v = InvocationError('/private/tmp/1/.tox/py27/bin/pip install --pre pyuv uzmq (see /private/tmp/1/.tox/py27/log/py27-1.log)', 1)
```

Start by reproducing the issue with the shown tox configuration, Python 2.7 environment, and dependencies pyuv and uzmq. Inspect the dependency-installation and setup.py egg_info failure, then verify that tox installs both packages and the configured command prints OK.

Written by the indexing model from the issue text.