config -- Avoid deleting existing dns section if someone removes schema_version from yaml file.

config.go

@@ -72,6 +72,7 @@ var config = configuration{
 		{Filter: dnsfilter.Filter{ID: 3}, Enabled: false, URL: "https://hosts-file.net/ad_servers.txt", Name: "hpHosts - Ad and Tracking servers only"},
 		{Filter: dnsfilter.Filter{ID: 4}, Enabled: false, URL: "http://www.malwaredomainlist.com/hostslist/hosts.txt", Name: "MalwareDomainList.com Hosts List"},
 	},
+	SchemaVersion: currentSchemaVersion,
 }
 
 // Loads configuration from the YAML file

upgrade.go

@@ -126,8 +126,10 @@ func upgradeSchema1to2(diskConfig *map[string]interface{}) error {
 		}
 	}
 
+	if _, ok := (*diskConfig)["dns"]; !ok {
 		(*diskConfig)["dns"] = (*diskConfig)["coredns"]
 		delete((*diskConfig), "coredns")
+	}
 	(*diskConfig)["schema_version"] = 2
 
 	return nil