fix 'rake gem' to work with early rubies
fix 'rake gem' to work with early rubies

For some reason s.files << x doesn't have any effect on the one Ruby 1.8.7
install I have at hand.

assispedro/leveldb-ruby,assispedro/leveldb-ruby,wmorgan/leveldb-ruby,assispedro/leveldb-ruby,wmorgan/leveldb-ruby,wmorgan/leveldb-ruby,wmorgan/leveldb-ruby,assispedro/leveldb-ruby