Run brakeman in a separate process

We were getting HAML errors; this is down to brakeman not being able
to keep track of every possible dependency and needing to be run in its
own process so as to not load the world (including HAML which is causing
the problem in this instance)

presidentbeef/brakeman#1044

lib/tasks/brakeman.rake

@@ -2,14 +2,7 @@ if Rails.env.development? || Rails.env.test?
   namespace :brakeman do
     desc 'Run Brakeman'
     task :run do
-      require 'brakeman'
-
-      Brakeman.run(
-        app_path: '.',
-        quiet: true,
-        pager: false,
-        print_report: true
-      )
+      `bundle exec brakeman --quiet --no-pager #{Rails.root}`
     end
   end
 end