BLD/TST: correct pytz version for tox
Tox fails to build pandas without a specific version of pytz.

hm wonder if this should be reported to upstream pytz? i seem to remember this working without a version at some point...
 Was failing for me, could be related to #4210?
 ah forgot about that...i don't follow `pip` dev, maybe i should
 FYI here's the error I get for attempting a clean build with tox without a version for pytz. 

It actually looks like tox is using pip 1.4 regardless of what version the local env is using (I've got pip 1.2 in the virtualenv I'm using). 

```
Downloading/unpacking pytz
  Could not find a version that satisfies the requirement pytz (from versions: 2009r, 2008b, 2009f, 2008c, 2007g, 2011g, 2005m, 2011e, 2007f, 2011k, 2007k, 2006j, 2008h, 2008i, 2011e, 2008a, 2009e, 2006g, 2011j, 2010l, 2005m, 2008i, 2005k, 2008g, 2007c, 2007i, 2009l, 2009r, 2006j, 2011k, 2007d, 2006p, 2009i, 2009u, 2007i, 2009f, 2010g, 2008h, 2009a, 2007g, 2011e, 2006p, 2012b, 2010k, 2005r, 2007f, 2009l, 2009p, 2008c, 2009j, 2008g, 2010g, 2010h, 2011h, 2010k, 2007c, 2007d, 2011d, 2009l, 2011c, 2008a, 2005m, 2007k, 2009n, 2011d, 2010o, 2013b, 2012h, 2010e, 2012c, 2012d, 2012f, 2011n, 2011b, 2011j, 2008c, 2012j, 2007k, 2009f, 2009d, 2010e, 2010b, 2013b, 2011d, 2009p, 2008h, 2005r, 2009i, 2009n, 2009a, 2010k, 2008g, 2006g, 2008b, 2012c, 2009i, 2007g, 2012c, 2010h, 2011n, 2012g, 2007d, 2008a, 2009u, 2012g, 2010o, 2006p, 2010b, 2009u, 2012d, 2011k, 2012f, 2009a, 2007f, 2011h, 2010l, 2009j, 2011g, 2009g, 2009g, 2005r, 2011c, 2012g, 2009g, 2012d, 2009j, 2010o, 2007c, 2010g, 2006g, 2009d, 2010h, 2005k, 2006j, 2010b, 2009n, 2011g, 2011c, 2012b, 2009e, 2009d, 2011j, 2007i, 2012j, 2010l, 2009r, 2012h, 2010e, 2009p, 2008i, 2012f, 2009e, 2012b, 2011h, 2005k, 2008b, 2013b, 2011n, 2012j, 2004b)
No distributions matching the version for pytz
```

```
(py27)kieran@smallbang ~/work/git/pandas $ pip --version
pip 1.2.1 from /home/kieran/work/pyenv/py27/lib/python2.7/site-packages/pip-1.2.1-py2.7.egg (python 2.7)
(py27)kieran@smallbang ~/work/git/pandas $ .tox/py27/bin/pip --version
pip 1.4 from /home/kieran/work/git/pandas/.tox/py27/lib/python2.7/site-packages (python 2.7)
```
 yep i get that too i just remember the 2012 pytz versions working w/o this...
 Also I'm using the latest tox

```
(py27)kieran@smallbang ~/work/git/pandas $ tox --version
1.5.0 imported from /home/kieran/work/pyenv/py27/lib/python2.7/site-packages/tox/__init__.pyc
```
 i also am really :-1: on this not removing of build directories when i try to update my env packages
 thanks @Komnomnomnom 
