flamingo: Modify settings.py after upstream API change

Signed-off-by: Chris Fiege <chris@tinyhost.de>

flamingo/settings.py

@@ -2,7 +2,6 @@ import os
 
 PLUGINS = [
     'flamingo.plugins.Redirects',
-    'flamingo.plugins.Layers',
     'flamingo.plugins.rstBootstrap3',
     'flamingo.plugins.rstFile',
     'flamingo.plugins.rstPygments',
@@ -15,6 +14,6 @@ PLUGINS = [
 # we can now use our own "Raw"
 DEFAULT_PLUGINS = [x for x in DEFAULT_PLUGINS if  x != "flamingo.plugins.HTML"]
 
-LAYERS = [
+POST_BUILD_LAYERS = [
         'static',
 ]