jquery-ui: switch to sources.buildroot.net

Switch to sources.buildroot.net cached zipfile.
Upstream changed the zipfile and the internal directory structure
without notifying anyone or changing the filename thus causing breakage.
And we can't use github release tarballs since they are not minified.
And we shouldn't reflect this in the install hook since an old cached
tarball wouldn't work, and we'd break anyone using the old package file
with the new one too. Thanks upstream! Fixes:
http://autobuild.buildroot.net/results/43a/43a16a44a26b38f4c6a2f352d8bfe5e07af6b2e3/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>