specify Java release target platform using rake-compiler option

Author: tagomoris

Rakefile

@@ -34,8 +34,7 @@ if RUBY_PLATFORM =~ /java/
     jars = ["#{jruby_home}/lib/jruby.jar"]
     ext.classpath = jars.map { |x| File.expand_path(x) }.join(':')
     ext.lib_dir = File.join(*['lib', 'msgpack', ENV['FAT_DIR']].compact)
-    ext.source_version = '1.8'
-    ext.target_version = '1.8'
+    ext.release = '8'
   end
 else
   require 'rake/extensiontask'