Taxonomy.silva2ncbi.rb bug fixed

The previous update introduced an error: Silva already has a
superkingdom rank, that conflicts with NCBI’s definition that
corresponds to Silva’s domain. Silva’s superkingdom is now interpreted
as “no rank”, and lives somewhere between domain and kingdom. The rank
“remarks” are also considered now, and taxa with remarks ‘w’ are
ignored. Taxtastic seems happy with this solution.
New patch option in Taxonomy.silva2ncbi.rb