Use offical ruby/debug gem

Pry is not maintaned anymore.

The debug gem some how calls `File.exist?` and we
expect that in elements definitions spec. Luckily we can
simply remove that, because we only need to stub `File.read`
for this spec.

Gemfile

@@ -37,7 +37,7 @@ group :development, :test do
     gem "bumpy"
     gem "yard"
     gem "redcarpet"
-    gem "pry-byebug"
+    gem "debug"
     gem "listen"
     gem "localeapp", "~> 3.0", require: false
     gem "dotenv", "~> 2.2"

spec/models/alchemy/element_spec.rb

@@ -233,7 +233,6 @@ module Alchemy
         let(:yaml) { "- name: :symbol" }
 
         before do
-          expect(File).to receive(:exist?).and_return(true)
           expect(File).to receive(:read).and_return(yaml)
         end
 
@@ -246,7 +245,6 @@ module Alchemy
         let(:yaml) { "- default: 2017-12-24" }
 
         before do
-          expect(File).to receive(:exist?).and_return(true)
           expect(File).to receive(:read).and_return(yaml)
         end
 
@@ -259,7 +257,6 @@ module Alchemy
         let(:yaml) { "- format: !ruby/regexp '/\A[^@\s]+@([^@\s]+.)+[^@\s]+\z/'" }
 
         before do
-          expect(File).to receive(:exist?).and_return(true)
           expect(File).to receive(:read).and_return(yaml)
         end