Bump underscore dependency.

Backbone 0.9.x happily works with underscore 1.4.x. There are some minor backwards incompatibilities in 1.4.0 & 1.4.1, but nothing that should impact B.M.H.

I see you're also the owner of this package on [jam](http://jamjs.org). If you could update this there, also, it'd be much appreciated. At the moment we have to roll our own version of this package with this one small change.