SQL: suppress warning for BIGINT with sqlite and sqlalchemy<0.8.2 (GH7433)

From discussion here: https://github.com/pydata/pandas/pull/7634#issuecomment-48111148 
Due to switching from Integer to BigInteger (to support int64 on some database systems), reading a table from sqlite with integers leads to a warning when you have an slqalchemy version of below 0.8.2.

I know it is very very late and goes against all reservations of putting in new stuff just before a release, but after some more consideration, I think we should include this (or at least something that fixes it, and I think this does). @jreback, you said not to worry about it (it is just a warning), but the sqlalchemy release that fixes it is only just a year old and this is something most users will try as the first thing I think when using the sqlalchemy functions (writing/reading simple dataframe with some numbers with sqlite).

I tested it locally with sqlalchemy 0.7.8 (on Windows), and on travis it is tested with 0.7.1 (the py2.6 build) and there also the warnings disappeared.