set fail_timeout=0 as recommended by Unicorn

Set's `fail_timeout=0` as recommended by http://unicorn.bogomips.org/Unicorn/Configurator.html#method-i-timeout when Unicorn is running behind nginx.

> For running Unicorn behind nginx, it is recommended to set `fail_timeout=0` for in your nginx configuration like this to have nginx always retry backends that may have had workers SIGKILL-ed due to timeouts.

From [Module ngx_http_upstream_module](http://nginx.org/en/docs/http/ngx_http_upstream_module.html#server):
> `fail_timeout=time` sets
* the time during which the specified number of unsuccessful attempts to communicate with the server should happen to consider the server unavailable;
* and the period of time the server will be considered unavailable.

> By default, the parameter is set to 10 seconds.

Courtesy of @phunehehe from https://github.com/gitlabhq/gitlabhq/issues/6621#issuecomment-44828050.