Fix small sinatra bug

Fix an issue with sinatra when no views folder is present.

Error:
```
$ dawn .
11:50:38 [*] dawn v1.2.0 is starting up
/Users/sadunne/.rvm/gems/ruby-2.0.0-p481@rest-server/gems/codesake-dawn-1.2.0/lib/codesake/dawn/sinatra.rb:37:in `detect_reflected_xss': undefined method `each' for nil:NilClass (NoMethodError)
	from /Users/sadunne/.rvm/gems/ruby-2.0.0-p481@rest-server/gems/codesake-dawn-1.2.0/lib/codesake/dawn/sinatra.rb:22:in `initialize'
	from /Users/sadunne/.rvm/gems/ruby-2.0.0-p481@rest-server/gems/codesake-dawn-1.2.0/lib/codesake/dawn/core.rb:107:in `new'
	from /Users/sadunne/.rvm/gems/ruby-2.0.0-p481@rest-server/gems/codesake-dawn-1.2.0/lib/codesake/dawn/core.rb:107:in `detect_mvc'
	from /Users/sadunne/.rvm/gems/ruby-2.0.0-p481@rest-server/gems/codesake-dawn-1.2.0/bin/dawn:167:in `<top (required)>'
	from /Users/sadunne/.rvm/gems/ruby-2.0.0-p481@rest-server/bin/dawn:23:in `load'
	from /Users/sadunne/.rvm/gems/ruby-2.0.0-p481@rest-server/bin/dawn:23:in `<main>'
	from /Users/sadunne/.rvm/gems/ruby-2.0.0-p481@rest-server/bin/ruby_executable_hooks:15:in `eval'
	from /Users/sadunne/.rvm/gems/ruby-2.0.0-p481@rest-server/bin/ruby_executable_hooks:15:in `<main>'
```