Drop trailing slash of prefix in #get_local_files

The local filesystem glob in AssetSync::Storage#get_local_files uses
fuzzy matching when config.prefix is present. This can present a problem
in some cases, as it doesn't allow for distinguishing between (e.g.) a
folder called assets/ and another folder called assets-temp/. A
situation could arise where the latter folder has thousands/millions of
files and we mistakenly publish local-only files, or worse we could
clobber another directory in the bucket managed in a completely
different context.

This change allows developers to be more specific in their
`config.prefix` by using a trailing slash for their folder name.

lib/asset_sync/storage.rb

@@ -140,7 +140,7 @@ def get_local_files
 
       log "Using: Directory Search of #{path}/#{self.config.assets_prefix}"
       Dir.chdir(path) do
-        to_load = self.config.assets_prefix.present? ? "#{self.config.assets_prefix}/**/**" : '**/**'
+        to_load = self.config.assets_prefix.present? ? File.join(self.config.assets_prefix, '/**/**') : '**/**'
         Dir[to_load]
       end
     end