Modifying gemspec to use grape v0.19.1

Author: Cassidy K

active_model_serializers.gemspec

@@ -57,7 +57,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'bundler', '~> 1.6'
   spec.add_development_dependency 'simplecov', '~> 0.11'
   spec.add_development_dependency 'timecop', '~> 0.7'
-  spec.add_development_dependency 'grape', ['>= 0.13', '< 1.0']
+  spec.add_development_dependency 'grape', ['>= 0.13', '< 0.19.1']
   spec.add_development_dependency 'json_schema'
   spec.add_development_dependency 'rake', ['>= 10.0', '< 12.0']
 end