Overwrite if server block exists

This is to prevent duplicate server blocks in a single file which gives errors when trying to restart nginx.

I was getting these errors because my server block files contained duplicate server blocks (with the oldest one on top).

```bash
vagrant@homestead:~$ nginx -s reload
nginx: [alert] could not open error log file: open() "/var/log/nginx/error.log" failed (13: Permission denied)
2014/05/22 12:00:02 [warn] 3172#0: the "user" directive makes sense only if the master process runs with super-user privileges, ignored in /etc/nginx/nginx.conf:1
2014/05/22 12:00:02 [warn] 3172#0: conflicting server name "dries.loc" on 0.0.0.0:80, ignored
2014/05/22 12:00:02 [warn] 3172#0: conflicting server name "dries.loc" on 0.0.0.0:80, ignored
2014/05/22 12:00:02 [warn] 3172#0: conflicting server name "dries.loc" on 0.0.0.0:80, ignored
2014/05/22 12:00:02 [warn] 3172#0: conflicting server name "dries.loc" on 0.0.0.0:80, ignored
2014/05/22 12:00:02 [warn] 3172#0: conflicting server name "dries.loc" on 0.0.0.0:80, ignored
2014/05/22 12:00:02 [warn] 3172#0: conflicting server name "dries.loc" on 0.0.0.0:80, ignored
2014/05/22 12:00:02 [warn] 3172#0: conflicting server name "lw.loc" on 0.0.0.0:80, ignored
2014/05/22 12:00:02 [warn] 3172#0: conflicting server name "lw.loc" on 0.0.0.0:80, ignored
2014/05/22 12:00:02 [warn] 3172#0: conflicting server name "lw.loc" on 0.0.0.0:80, ignored
2014/05/22 12:00:02 [warn] 3172#0: conflicting server name "lw.loc" on 0.0.0.0:80, ignored
2014/05/22 12:00:02 [notice] 3172#0: signal process started
2014/05/22 12:00:02 [alert] 3172#0: kill(3079, 1) failed (1: Operation not permitted)    
```