Fix leak of file handles that prevents gzip files from being cleaned up

Use `Zlib::GzipWriter::new(IO)` instead of `Zlib::GzipWriter::open(Object)`, since
the latter effectively creates an additional file handle when passed an `IO`. This
ensures that when we send `Zlib::GzipWriter#close`, the original `IO` is closed in
turn.

Resolves: #190

lib/logstash/outputs/s3/temporary_file_factory.rb

@@ -100,7 +100,7 @@ class IOWrappedGzip
 
           def initialize(file_io)
             @file_io = file_io
-            @gzip_writer = Zlib::GzipWriter.open(file_io)
+            @gzip_writer = Zlib::GzipWriter.new(file_io)
           end
 
           def path