Build: Specify jQuery version as >=1.6, do custom install for this repo

The new bowerjquery task installs the jQuery version as specified in
devDependencies, while the dependencies field just specifies the minimal
version required.

Fixes [#10110](http://bugs.jqueryui.com/ticket/10110)

Probably a bad idea to work on this before #1266 lands, but then I didn't change much in `Gruntfile.js`.
It seems like we should either declare a specific version here or just go with `"<2"`.

On a related note, maybe I don't know what I'm doing, but when I update this to `"jquery": ">=1.6 < 1.12"`, and run `grunt bower` on this branch, nothing happens. It seems like I should get 1.11.1.