Disable unstable delimiter test

Upstream-Issue: https://github.com/gjtorikian/mathematical/issues/73
--- a/test/mathematical/delimiters_test.rb
+++ b/test/mathematical/delimiters_test.rb
@@ -46,6 +46,7 @@
   end
 
   def test_mixed
+    skip 'Disabled, because it randomly fails (#73)'
     render = Mathematical.new(:delimiter => [:brackets, :double], :format => :mathml)
 
     fixture_tex = "\\[\\alpha\\] $$\\beta$$"