Follow shell command guidelines in Brakeman task

There is no real issue here because the shell command is completely
static, but it is still nicer to just follow the guidelines in
doc/development/shell_commands.md .

lib/tasks/brakeman.rake

 desc 'Security check via brakeman'
 task :brakeman do
-  if system("brakeman --skip-files lib/backup/repository.rb -w3 -z")
+  if system(*%W(brakeman --skip-files lib/backup/repository.rb -w3 -z))
     puts 'Security check succeed'
   else
     puts 'Security check failed'