.gitmodules: update giflib remote uri (#1749)
.gitmodules: update giflib remote uri (#1749)

sourceforge has been performing a datacenter migration, which resulted
in a service disruption.
Access via https:// is still broken, but via git:// it is working again.
pagespeed/mod_pagespeed,pagespeed/mod_pagespeed,pagespeed/mod_pagespeed,pagespeed/mod_pagespeed,pagespeed/mod_pagespeed,pagespeed/mod_pagespeed,pagespeed/mod_pagespeed