Fix broken expansion of asset link paths

This fixes a regression from 36741c6 where a line was missed in the
conversion.

Fixes #613

lib/sprockets/loader.rb

@@ -115,6 +115,7 @@ def asset_from_cache(key)
           asset[:load_path] = expand_from_root(asset[:load_path])
           asset[:filename]  = expand_from_root(asset[:filename])
           expand_key_from_hash(asset[:metadata], :included)
+          expand_key_from_hash(asset[:metadata], :links)
           expand_key_from_hash(asset[:metadata], :stubbed)
           expand_key_from_hash(asset[:metadata], :required)
           expand_key_from_hash(asset[:metadata], :to_load)