Update swagger validate to use correct cli command (#835)

Author: calderwoodra

lib/grape-swagger/rake/oapi_tasks.rb

@@ -64,7 +64,7 @@ def validate
           ::Rake::Task['oapi:fetch'].invoke
           exit if error?
 
-          output = system "swagger validate #{file}"
+          output = system "swagger-cli validate #{file}"
 
           $stdout.puts 'install swagger-cli with `npm install swagger-cli -g`' if output.nil?
           FileUtils.rm(file)