COMPAT: change pytables to use 3.0.0 API (GH7990)
closes #7990 

nice: - is bigger than +
 that's a first!
 @cpcloud @jorisvandenbossche comments?
 minor comments. otherwise looks good
 Would this be annoying then? look at the tests

https://travis-ci.org/jreback/pandas/jobs/32325575
 I think it is a useful and informative message, unless you don't bother to use numexpr (and it is eg installed in your distribution) ... then it can become annoying (but that is always like this with warnings no?). 
I just looked, I have eg still numexpr 2.0.1 (installed a time ago with anaconda), and I suppose I am not alone (but with conda of course easy to upgrade).
 What is reason for upgrading the required versions? Is it only testing? (that we have to test less versions?) Because both PyTables 3 and Numexpr 2.1 are only 1 to 1.5 year old.
 @jorisvandenbossche yes < 2.1 is pretty old at this point.

The reason for the changes is motiviated by using conda for ci, as it doesn't support these (pretty old) versions.

And we have to do this at some point before 3.2 PyTables is out (maybe end of year). As the api will start showing a different kind of warning. (it shows DeprecationWarning on all tests atm, but we are suppressing this).

I don't think this is really a big deal. 99% of users dumped 2.4 as soon as 3.0.0 came out because it supports py3 (and had a lot of bug fixes). (and its backward compatible).
 @jreback OK, sounds reasonable
 @jreback I got the warning :-) 
But I now got: `"UserWarning: The installed version of numexpr 2.0.1 is not supported in pandas and will be not be used"` -> maybe add what version it should be at least?
 hmm, was 2.0.0, now its 2.1, ok easy to fix
 https://github.com/pydata/pandas/commit/2594bea462dcfa55eaaf76ed1b3e922443a81f82
 Thanks!
