Restructure dependencies

Move them to Gemfile

Gemfile

@@ -5,5 +5,8 @@ source 'https://rubygems.org'
 # Specify your gem's dependencies in planet_express.gemspec
 gemspec
 
-gem 'rake', '~> 12.0'
-gem 'rspec', '~> 3.0'
+gem 'rake'
+gem 'rspec'
+gem 'factory_bot'
+gem 'rubocop'
+gem 'simplecov'

planet_express.gemspec

@@ -29,7 +29,5 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ['lib']
 
-  spec.add_development_dependency('factory_bot', '~> 6.1')
-  spec.add_development_dependency('rubocop', '~> 1.11')
-  spec.add_development_dependency('simplecov', '~> 0.21')
+
 end