use rake < 11 since there are some known issues currently with for ex…

…ample rubocop (rubocop/rubocop#2931) that will give you this error:

```

MethodError: undefined method `last_comment' for #<Rake::Application
```

spree_social.gemspec

@@ -44,4 +44,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'sass-rails', '~> 5.0.0'
   s.add_development_dependency 'guard-rspec'
   s.add_development_dependency 'rubocop', '>= 0.24.1'
+  s.add_development_dependency 'rake', '<11'
 end