chore: pin dev deps to address gem build warnings

Fixes #100

Author: brettchalupa

.rubocop.yml

@@ -1,6 +1,4 @@
-inherit_gem:
-  rubocop-standard:
-    - config/default.yml
+require: rubocop-performance
 
 Style/StringLiterals:
   Enabled: true

graphql-docs.gemspec

@@ -48,10 +48,9 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'html-proofer', '~> 3.4'
   spec.add_development_dependency 'minitest', '~> 5.0'
   spec.add_development_dependency 'minitest-focus', '~> 1.1'
-  spec.add_development_dependency 'rake'
-  spec.add_development_dependency 'rubocop'
-  spec.add_development_dependency 'rubocop-performance'
-  spec.add_development_dependency 'rubocop-standard'
+  spec.add_development_dependency 'rake', '~> 13.0'
+  spec.add_development_dependency 'rubocop', '~> 1.37'
+  spec.add_development_dependency 'rubocop-performance', '~> 1.15'
   spec.add_development_dependency 'webmock', '~> 2.3'
   spec.add_development_dependency 'webrick', '~> 1.7'
 end