Remove deprecated usage of scope attribute

[Phobetor]

Fixes these deprecation messages / incompatibilities with symfony 3.0:

• The "scope" attribute of service "fos_comment.spam_detection.comment.akismet" in file "FOS/CommentBundle/Resources/config/spam_detection.xml" is deprecated since version 2.8 and will be removed in 3.0
• The "scope" attribute of service "fos_comment.listener.comment_spam" in file "FOS/CommentBundle/Resources/config/spam_detection.xml" is deprecated since version 2.8 and will be removed in 3.0

[XWB]

:+1: Thanks.