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

The new bowerinstall 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`.
We actually want an explicit version here, not a range. We want to manually upgrade as new versions come out so we can make sure we fix any potential problems before switching to the new version.