Merge pull request #72 from adzuci/patch-1

Update yard to 0.9.20

Author: benedikt

mongoid-tree.gemspec

@@ -16,5 +16,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency('mongoid-compatibility')
   s.add_development_dependency('rake', ['>= 0.9.2'])
   s.add_development_dependency('rspec', ['~> 3.0'])
-  s.add_development_dependency('yard', ['~> 0.8'])
+  s.add_development_dependency('yard', ['>= 0.9.20'])
 end