Change warning level from verbose to medium

Suppresses thousands of instance variable not initialized warnings from
rdf-1.0.8 gem.  NCBO-1725.

Author: jvendetti

Rakefile

@@ -3,6 +3,7 @@ require 'rake/testtask'
 Rake::TestTask.new do |t|
   t.libs = []
   t.test_files = FileList['test/**/test*.rb']
+  t.ruby_opts = ['-W1']
 end
 
 def clear_cache