build: use non-deprecated compiler flag when building jruby extension (#202)

The current build uses deprecated Java compiler flags which do not fully
guarantee runtime compatibility with the targeted Java runtime version.

it doesn't cause any issues right right as the Java extension code is
limited; but this would be better practice; especially as builds are
executed with a non-locked Java runtime determined by `setup-ruby`
defaults.

Author: chadlwilson

Rakefile

@@ -11,8 +11,7 @@ when "jruby"
   require 'rake/javaextensiontask'
   extask = Rake::JavaExtensionTask.new("stringio") do |ext|
     ext.lib_dir << "/#{ext.platform}"
-    ext.source_version = '1.8'
-    ext.target_version = '1.8'
+    ext.release = '8'
     ext.ext_dir = 'ext/java'
   end
   libs = [extask.lib_dir]