Replace pry-nav with pry-byebug

pry-nav works, but produces a "circular require" warning.
The warning was fixed in 2016, but no new version of pry-nav
has been released.

https://github.com/nixme/pry-nav/pull/26

In fact, the pry-nav readme recommends using pry-byebug.

paper_trail.gemspec

@@ -40,7 +40,7 @@ has been destroyed.
   s.add_development_dependency "rspec-rails", "~> 3.5"
   s.add_development_dependency "generator_spec", "~> 0.9.3"
   s.add_development_dependency "database_cleaner", "~> 1.2"
-  s.add_development_dependency "pry-nav", "~> 0.2.4"
+  s.add_development_dependency "pry-byebug", "~> 3.4"
   s.add_development_dependency "rubocop", "0.48.0"
   s.add_development_dependency "rubocop-rspec", "~> 1.15.0"
   s.add_development_dependency "timecop", "~> 0.8.0"

spec/spec_helper.rb

@@ -1,4 +1,4 @@
-require "pry-nav"
+require "pry"
 
 # This file was generated by the `rspec --init` command. Conventionally, all
 # specs live under a `spec` directory, which RSpec adds to the `$LOAD_PATH`.

test/test_helper.rb

@@ -1,4 +1,4 @@
-require "pry-nav"
+require "pry"
 
 ENV["RAILS_ENV"] = "test"
 ENV["DB"] ||= "sqlite"