CI: Drop numpy 1.6 support
closes #7711 

well that was cathartic
 fyi, your 2.6/2.7_locale build are prob taking forever as they have to compie numpy (until the wheel is there)
 yeah i know .. there's some fails too
 i think we might need to require 1.7.1, there's an issue with type coercion in ufuncs in 1.7.0 not totally sure what the issue is taking a look now
 arg nope 1.7.1 still fails
 ok wheels are up for 1.7.0/1.7.1 on both 2.6 and 2.7_LOCALE

enjoy!
 I'm going to close this until i make some more progress
 whoops wrong pr
 This will pass but there's a weird bug where 

``` python
with tm.assertRaises(ValueError):
    o.item()
```

will fail but

``` python
self.assertRaises(ValueError, o.item)
```

will pass

there might be a weird py26 bug in this decorator ... let me see if i can't dig around and fix it
 @jreback @jorisvandenbossche Any comments? The assert raises issue only shows up on Travis, even when I use the exact same deps. 
 yeh I think their might be something in the decorator

I always use:

`self.assertRaises(ValueError, lambda : o.item())`
 need to update `install.rst` and big fat warning in v0.15.0 and highlites (also may need to update else where in docs)
 Cool. Will do. 

You actually don't need a lambda in those cases because the function is already a no argument callable. A bit easier to read IMO 
 @jreback good to go?
 i'll squash if you watn
 version is also mentioned in the readme
 minor comments (I would squash). ok otherwise

@jorisvandenbossche 
 alright if this next build passes going to merge

@jorisvandenbossche thanks for the heads up about the readme
 ok sure
 this is passing on my fork, travis is going to take forever to get to this i guess can merge tomorrow
 if passing then ok to merge
