Add activemodel-serializers-xml to Gemfile

Rails 5 removed these features from core, instead using a seperate gem.

rails/rails#21161

Author: bagedevimo

Gemfile

@@ -43,6 +43,8 @@ gem "ruby-duration"
 gem "pg"
 gem "backup"
 
+gem 'activemodel-serializers-xml'
+
 # Redis and Background Processing
 gem "redis"
 gem "qless", github: "Shopify/qless"

Gemfile.lock

@@ -45,6 +45,10 @@ GEM
       globalid (>= 0.3.6)
     activemodel (5.0.7.2)
       activesupport (= 5.0.7.2)
+    activemodel-serializers-xml (1.0.3)
+      activemodel (>= 5.0.0.a)
+      activesupport (>= 5.0.0.a)
+      builder (~> 3.1)
     activerecord (5.0.7.2)
       activemodel (= 5.0.7.2)
       activesupport (= 5.0.7.2)
@@ -421,6 +425,7 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
+  activemodel-serializers-xml
   backup
   better_errors
   binding_of_caller