Set twig strict_variables config to suppress the deprecation notices (#237)

This is a behavioural change: the default value is 'false' compared to the new default value of '%kernel.debug%' which eventually evaluates to true when running the tests.
Since it seems that this configuration is only used in the functional tests, there should be nothing functionally breaking in the bundle

Author: nstapelbroek

Tests/Functional/app/config/framework.yml

@@ -14,5 +14,6 @@ framework:
         engines: ['twig']
 
 twig:
+  strict_variables: "%kernel.debug%" #supresses deprecation notices about the default value TwigBundle pre version 5
   paths:
     "%test.root_dir%/Resources/views": App