[Chore] Use Gemfile instead of gemfiles/all.gemfile

It's hard to maintain both and run `bundle install` for all file.
We also need `dry-inflector` for testing.

Gemfile

@@ -4,6 +4,7 @@ source 'https://rubygems.org'
 gemspec
 
 gem 'activesupport', require: false # For backend
+gem 'dry-inflector', require: false # For inflection
 gem 'ffaker', require: false # For testing
 gem 'inch', require: false # For inline documents
 gem 'minitest', '~> 5.14' # For test

Rakefile

@@ -1,12 +1,12 @@
 require "bundler/gem_tasks"
 require "rake/testtask"
 
-ENV["BUNDLE_GEMFILE"] = File.expand_path("gemfiles/all.gemfile") if ENV["BUNDLE_GEMFILE"] == File.expand_path("Gemfile") || ENV["BUNDLE_GEMFILE"].empty? || ENV["BUNDLE_GEMFILE"].nil?
+ENV["BUNDLE_GEMFILE"] = File.expand_path("Gemfile") if ENV["BUNDLE_GEMFILE"] == File.expand_path("Gemfile") || ENV["BUNDLE_GEMFILE"].empty? || ENV["BUNDLE_GEMFILE"].nil?
 
 Rake::TestTask.new(:test) do |t|
   t.libs << "test"
   t.libs << "lib"
-  file_list = ENV["BUNDLE_GEMFILE"] == File.expand_path("gemfiles/all.gemfile") ? FileList["test/**/*_test.rb"] : FileList["test/dependencies/test_dependencies.rb"]
+  file_list = ENV["BUNDLE_GEMFILE"] == File.expand_path("Gemfile") ? FileList["test/**/*_test.rb"] : FileList["test/dependencies/test_dependencies.rb"]
   t.test_files = file_list
 end