Update numpy version to work with this version of pandas.

[robin-norwood]

On my MacOS 10.11.5 installation running python3, (not using anaconda), I got the following error:

$ virtualenv venv
$ source venv/bin/activate
$ python --version
$ python --version
Python 3.5.1
$ pip install -r requirements.txt
...
$ python nsfg.py 
numpy.dtype has the wrong size, try recompiling

After the included change:

$ virtualenv venv
$ source venv/bin/activate
$ pip install -r requirements.txt
...
$ python nsfg.py 
(13593, 244)
nsfg.py: All tests passed.

I haven't gotten to other examples from the book, so I'm not sure if anything else is broken. If so, I'll just switch to using anaconda instead.

[AllenDowney]

Thanks!