Skip test_race_exception on MJIT for now

somehow the test fails only on Wercker in CIs triggered by GitHub.
Actually rubyci and ci.rvm.jp also have issues too, though.

test/ruby/test_require.rb

@@ -503,6 +503,7 @@ class TestRequire < Test::Unit::TestCase
   end
 
   def test_race_exception
+    skip 'require has had a race condition since 5931857281ce45c1c277aa86d1588119ab00a955' if RubyVM::MJIT.enabled?
     bug5754 = '[ruby-core:41618]'
     path = nil
     stderr = $stderr