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`.
I've removed the separate install command I had earlier, since the bower behaviour apparently is here to stay, always installing project dependencies even when a specific endpoint is specified.