Fix go modules

```
go get github.com/ip2location/[email protected]
go get: github.com/ip2location/[email protected]: invalid version: module contains a go.mod file, so major version must be compatible: should be v0 or v1, not v3
```

Author: un000

go.mod

@@ -1,3 +1,3 @@
-module github.com/ip2location/ip2proxy-go
+module github.com/ip2location/ip2proxy-go/v3
 
 go 1.14