Pin rake to < 11.0

https://stackoverflow.com/questions/35893584/nomethoderror-undefined-method-last-comment-after-upgrading-to-rake-11/35893625#35893625

Author: remear

active_model_serializers.gemspec

@@ -56,4 +56,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'minitest-reporters'
   spec.add_development_dependency 'grape', ['>= 0.13', '< 1.0']
   spec.add_development_dependency 'json_schema'
+  # https://stackoverflow.com/questions/35893584/nomethoderror-undefined-method-last-comment-after-upgrading-to-rake-11/35893625#35893625
+  spec.add_development_dependency 'rake', '< 11.0'
 end