Update rspec-rails dependency

Bump the requirement to >= 4.0 (which supports Rails >= 4.2) to allow usage
of 6.x version in modern Rails

Close jsonapi-rb#142

Author: tagliala

jsonapi-rails.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'rails', '>= 5.0'
   spec.add_development_dependency 'sqlite3', '>= 1.3'
   spec.add_development_dependency 'rake',        '>= 11.3'
-  spec.add_development_dependency 'rspec-rails', '~> 3.5'
+  spec.add_development_dependency 'rspec-rails', '>= 4.0'
   spec.add_development_dependency 'with_model',  '~> 2.0'
   spec.add_development_dependency 'simplecov'
 end