Corrected set_cookie for Sinatra > 0.9.4

[conatus]

Attempted to get this running but found that cookie wasn't being set. Turns out Sinatra 0.9.4 does things a little bit differently. Corrected this in main.rb.

Great program, by the way.

[mgriffin]

I had just done the same edit right before I saw this request.

[kiruri]

Hi! I'm using Sinatra 1.1.0.

When I try to login, I get:
undefined method `set_cookie' for #Sinatra::Application:0x7f9fa41708b8

[chriswaugh]

+1 Also made the same change locally - will refrain from committing it in the hope this is approved :)