[css-flexbox] Correction to flex-item auto in spec

The spec[1] says the following for implied minimum size of flex items for the keyword of 'auto':

     On a flex item whose overflow is _not_ visible...

This should probably read:

     On a flex item whose overflow _is_ visible

In its current state this would result in any flex-item that is set to overflow: visible to have its min-width/min-height set to 0

This seems to be the opposite of what Elliot was wanting[2]:

     # instead I think we should combine the behavior so the min-width is min-content unless your overflow
     # property computes to a value other than visible in which case it should be 0.

The explanation to Elliot for resolving on this seems[3] to imply that the spec is aligning with IEs implementation so this seems to be a typo. Sorry we didn't catch this earlier.

Thanks,
Greg

[1] http://dev.w3.org/csswg/css-flexbox/#min-size-auto
[2] http://lists.w3.org/Archives/Public/www-style/2013Apr/0538.html
[3] http://lists.w3.org/Archives/Public/www-style/2014Mar/0463.html