Fixate simplecov dependency

Simplecov doesn't support Ruby 2.4 in newer versions
and the latest possible version generates reports
which are not processable by the code climate reporter.

So it seems we're stuck with simplecov 0.17.1 until
Ruby 2.4 is dropped:

codeclimate/test-reporter#413 (comment)

draper.gemspec

@@ -30,5 +30,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'capybara'
   s.add_development_dependency 'active_model_serializers', '>= 0.10'
   s.add_development_dependency 'rubocop'
-  s.add_development_dependency 'simplecov'
+  s.add_development_dependency 'simplecov', '0.17.1'
 end