Replace pry and pry-byebug with debug in development dependencies (#438)

I don't have a strong opinion, but in most cases I think it is better to use what is provided by Ruby org.
https://github.com/ruby/debug

Author: ydah

committee.gemspec

@@ -28,8 +28,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "minitest", "~> 5.3"
   s.add_development_dependency "rack-test"
   s.add_development_dependency "rake", "~> 13.1"
-  s.add_development_dependency "pry"
-  s.add_development_dependency "pry-byebug"
+  s.add_development_dependency "debug"
   s.add_development_dependency "rubocop"
   s.add_development_dependency "rubocop-performance"
   s.add_development_dependency "simplecov"

test/test_helper.rb

@@ -21,7 +21,7 @@
 require "minitest/spec"
 require "minitest/autorun"
 require "rack/test"
-require "pry"
+require "debug"
 require "stringio"
 
 require_relative "../lib/committee"