add brakeman security scanner to guard

Guardfile

@@ -78,8 +78,16 @@ guard :shell, all_on_start: true do
   end
 end
 
-guard :rubocop do
+guard :rubocop, cli: %w(-D -S) do
+  watch(/.rubocop.yml/)
   watch(/.+\.rb$/)
   watch(/Rakefile/)
   watch(%r{(?:.+/)?\.rubocop\.yml$}) { |m| File.dirname(m[0]) }
 end
+
+guard 'brakeman', run_on_start: true, quiet: true do
+  watch(%r{^app/.+\.(erb|haml|rhtml|rb)$})
+  watch(%r{^config/.+\.rb$})
+  watch(%r{^lib/.+\.rb$})
+  watch('Gemfile')
+end