Best advice I could provide if the devs aren't too knowledgeable on application security, is use a framework where possible, know where it's likely to fail you and focus there and then make sure you stay on top of patching all your framework components to catch vulns in that layer
So for example rails handles basic things like SQL injection, XSS in standard forms fine, then you can use devise or similar for authentication and get a reasonable level of security. What you're left with are areas like authorisation which tends to be app. specific so still requires work, but you probably have less to focus on.
Also watch out for what I'd call "dangerous" functionality, things like file uploads or user generated content where you want users to enter HTML tags but still avoid XSS. Things like that need specific consideration to avoid common security issues.
Of course if you're doing something that will attract real bad guys (anything to do with payments, anything to do with bitcoin, anything to do with Intellectual property management etc) then I'd strongly recommend getting an app security person on staff as soon as you can 'cause you will get attacked probably sooner rather than later...