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'm fine with changing this version. I'm okay with `"<2"`, but I don't want to mix the jQuery update with this change.

Regarding the other problem, I've added a comment about that in the Gruntfile, where the path has to change. I also [reported the poor error handling](https://github.com/timmywil/grunt-bowercopy/issues/18).