Use low-level IO#syswrite instead of IO#write in File adapter

Since

1. we are using binmode and do not expect any conversation of newlines
   or encodings to take place, and
2. we are only writing to the open file when we have squired the mutex
   of the adapter instance

we can safely use the syswrite method to write logs. In the end, this
uses the same low-level syscall as `IO#write` but with less overhead.

lib/rackstash/adapter/file.rb

@@ -129,7 +129,7 @@ module Rackstash
 
         @mutex.synchronize do
           auto_reopen
-          @file.write(line)
+          @file.syswrite(line)
         end
         nil
       end
@@ -180,8 +180,7 @@ module Rackstash
 
         file = ::File.new(
           filename,
-          ::File::WRONLY | ::File::APPEND | ::File::CREAT,
-          external_encoding: Encoding::UTF_8
+          ::File::WRONLY | ::File::APPEND | ::File::CREAT
         )
         file.binmode
         file.sync = true