Don't use :redis_store in development environment

This commit fixes small problem. The problem is if you try to start gitlabhq application in development environment with non-default `redis`  port (e. g. it's possible if you use @boxen) you will have `Rack` exception (`Redis on 127.0.0.1:6379 (ECONNREFUSED)`).

Improper `session_store` configuration causes such a problem.