Point directly at the sourceforge download for icu

The icu project has archived this version, so it now redirects to sourceforge,

this causes a http to https redirect issue with omnibus

In the past we have dealt with this by updating icu, (the project hosts the latest archive on their website, and redirects to sourceforge for the rest)

But the newest libicu requires C++11 support which currently won't work on some of our older platforms. So in order to stick with our current version, we can point directly at the sourceforge download.