style: specify development dependencies in Gemfile

as per rubocop recommendation.
I wondered if that brings any practical benefits, but it seems to be purely about consistency
rubocop/rubocop#11469 (comment)

Gemfile

@@ -4,3 +4,11 @@ source 'https://rubygems.org'
 
 # Specify your gem's dependencies in morandi.gemspec
 gemspec
+
+gem 'guard'
+gem 'guard-rspec'
+gem 'rake'
+gem 'rspec'
+gem 'rubocop'
+gem 'simplecov'
+gem 'super_diff'

morandi.gemspec

@@ -28,12 +28,4 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'gdk_pixbuf2', '> 4.0.0'
   spec.add_dependency 'pango', '> 4.0.0'
   spec.add_dependency 'rake-compiler'
-
-  spec.add_development_dependency 'guard'
-  spec.add_development_dependency 'guard-rspec'
-  spec.add_development_dependency 'rake'
-  spec.add_development_dependency 'rspec'
-  spec.add_development_dependency 'rubocop'
-  spec.add_development_dependency 'simplecov'
-  spec.add_development_dependency 'super_diff'
 end