Only accept literals as before_filter method names

whitelist, not blacklist!

This fixes an issue where things like local variables
were being interpreted as method names.

Example:

before_filter :blah, filter_options

"filter_options" was being treated as if it were a method name,
which was wrong.

lib/brakeman/processors/controller_alias_processor.rb

@@ -254,7 +254,7 @@ class Brakeman::ControllerAliasProcessor < Brakeman::AliasProcessor
     filter[:methods] = [args[0][1]]
 
     args[1..-1].each do |a|
-      filter[:methods] << a[1] unless a.node_type == :hash
+      filter[:methods] << a[1] if a.node_type == :lit
     end
 
     if args[-1].node_type == :hash