Fix Travis builds for MongoMapper 0.13 and 0.14

Author: laserlemon

gemfiles/mongo_mapper/0-13.gemfile

@@ -5,6 +5,7 @@ gem "mongo_mapper", "~> 0.13.0"
 gemspec name: "periscope-mongo_mapper", path: "../../"
 
 group :test do
+  gem "activemodel-serializers-xml"
   gem "codeclimate-test-reporter", "~> 1.0"
   gem "database_cleaner"
   gem "factory_girl"

gemfiles/mongo_mapper/0-14.gemfile

@@ -5,6 +5,7 @@ gem "mongo_mapper", "~> 0.14.0"
 gemspec name: "periscope-mongo_mapper", path: "../../"
 
 group :test do
+  gem "activemodel-serializers-xml"
   gem "codeclimate-test-reporter", "~> 1.0"
   gem "database_cleaner"
   gem "factory_girl"

spec/spec_helper.rb

@@ -8,6 +8,8 @@
 
 require "periscope"
 
+Bundler.require(:test)
+
 Dir["./spec/shared/*.rb"].shuffle.each { |f| require f }
 Dir["./spec/support/*.rb"].shuffle.each { |f| require f }