BLD: Change pytables to tables in the asv dependencies.
The asv benchmarks fails to build due to [PyTables PyPI package name being `tables` and not `pytables`](https://pypi.python.org/pypi/tables). This PR fixes the issue and the asv benchmarks now builds correctly.

this is meant for conda not thru pip
so I don't think this will work 
 I see. I think noting in [CONTRIBUTING.md](https://github.com/pydata/pandas/blob/master/CONTRIBUTING.md) that conda is the default would help. I was confused by the remark to use either `conda` or `virtualenv` with asv.
 yeh glad to take a PR to clarify this a bit

and/or alternate asv config file that only uses pip
