add rails/upgrade_5_1_to_5_2 snippet

lib/rails/upgrade_5_0_to_5_1.rb

@@ -2,6 +2,8 @@
 
 Synvert::Rewriter.new 'rails', 'upgrade_5_0_to_5_1' do
   description <<~EOS
+    It upgrades rails 5.0 to 5.1.
+
     1. it replaces `HashWithIndifferentAccess` with `ActiveSupport::HashWithIndifferentAccess`.
 
     2. it replaces `Rails.application.config.secrets[:smtp_settings]["address"]` with

lib/rails/upgrade_5_1_to_5_2.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+Synvert::Rewriter.new 'rails', 'upgrade_5_1_to_5_2' do
+  description <<~EOS
+    It upgrades rails 5.1 to 5.2
+
+    1. it replaces `dalli_store` with `mem_cache_store`
+  EOS
+
+  if_gem 'rails', { gte: '5.2.0' }
+
+  within_file 'config/application.rb' do
+    # dalli_store => mem_cache_store
+    with_node type: 'sym', to_value: 'dalli_store' do
+      replace_with ':mem_cache_store'
+    end
+  end
+end

lib/rails/upgrade_5_2_to_6_0.rb

@@ -1,8 +1,8 @@
 # frozen_string_literal: true
 
-# TODO: do more
 Synvert::Rewriter.new 'rails', 'upgrade_5_2_to_6_0' do
   description <<-EOS
+    It upgrades rails 5.2 to 6.0
   EOS
 
   add_snippet 'rails', 'convert_update_attributes_to_update'

spec/rails/upgrade_5_1_to_5_2_spec.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'Upgrade rails from 5.1 to 5.2' do
+  let(:rewriter_name) { 'rails/upgrade_5_1_to_5_2' }
+  let(:application_content) { "
+module Synvert
+  class Application < Rails::Application
+    config.cache_store = :dalli_store, 'cache-1.example.com', 'cache-2.example.com'
+  end
+end
+  "}
+  let(:application_rewritten_content) { "
+module Synvert
+  class Application < Rails::Application
+    config.cache_store = :mem_cache_store, 'cache-1.example.com', 'cache-2.example.com'
+  end
+end
+  "}
+  let(:fake_file_paths) { ['config/application.rb'] }
+  let(:test_contents) { [application_content] }
+  let(:test_rewritten_contents) { [application_rewritten_content] }
+
+  include_examples 'convertable with multiple files'
+end