Add realIp override to allow anonymous submissions

[Glorf]

This small fix should unblock the possibility of anonymous submissions (one per IP) in pybossa.
(fix for: #25)
The effectiveness of this fix is related to:

• How is the X-Forwarded-For formed by your loadbalancer
• What are IP(s) of your loadbalancer (I assume it's 192.172.189.251? - that's an IP attached to task exports for anonymous user. but it's unusual since it's a public address) (EDIT: I cannot see it, the real IP is anonymized by yacryptopan. You have to fill the relevant address/subnet yourselves)

I cannot help much more without knowing the internal network topology and seeing the logs, but this fix might be enough, please let me now if it worked

[janCstoffregen]

as discussed this enhancement is placed on the pybossa and nginx containers. thank you very very much again!