Upgrade to the latest AMS 0.8.x (unreleased)

The current released version (0.8.2) does not work with Rails master at all. In
fact, it was quite surprising to me that this is the only test that broke...

See https://github.com/rails-api/active_model_serializers/pull/655

Gemfile

@@ -91,7 +91,11 @@ gem 'redis', require:  ["redis", "redis/connection/hiredis"]
 # We use some ams 0.8.0 features, need to amend code
 # to support 0.9 etc, bench needs to run and ensure no
 # perf regressions
+if rails_master?
+  gem 'active_model_serializers', github: 'rails-api/active_model_serializers', branch: '0-8-stable'
+else
   gem 'active_model_serializers', '~> 0.8.0'
+end
 
 
 gem 'onebox'

Gemfile_master.lock

@@ -1,3 +1,11 @@
+GIT
+  remote: git://github.com/rails-api/active_model_serializers.git
+  revision: b6b01d0b7396f3deaa6e661cedf4bc5efe2f4525
+  branch: 0-8-stable
+  specs:
+    active_model_serializers (0.8.2)
+      activemodel (>= 3.0)
+
 GIT
   remote: https://github.com/rails/rails.git
   revision: aa6637d140c2ebd28bbd23fc250af033a065dbe8
@@ -74,8 +82,6 @@ GEM
   specs:
     actionpack-action_caching (1.1.1)
       actionpack (>= 4.0.0, < 5.0)
-    active_model_serializers (0.8.2)
-      activemodel (>= 3.0)
     addressable (2.3.6)
     annotate (2.6.5)
       activerecord (>= 2.3.0)
@@ -437,7 +443,7 @@ PLATFORMS
 
 DEPENDENCIES
   actionpack-action_caching
-  active_model_serializers (~> 0.8.0)
+  active_model_serializers!
   annotate
   barber
   better_errors