🐞 Import module xip correctly

Don't use the GitHub path; use the local directory.

See
<https://github.com/golang/go/wiki/Modules#do-modules-work-with-relative-imports-like-import-subdir>
for more information. tldr:

```golang
import "xip/xip"
```

The first `xip` is the module name, and the second `xip` is the
subdirectory. They happen to be the same in this case.

bosh-release/src/main.go

@@ -4,7 +4,7 @@ import (
 	"log"
 	"net"
 
-	"github.com/cunnie/sslip.io/src/xip"
+	"xip/xip"
 )
 
 func main() {

bosh-release/src/xip/xip_test.go

@@ -3,7 +3,7 @@ package xip_test
 import (
 	"math/rand"
 
-	"github.com/cunnie/sslip.io/src/xip"
+	"xip/xip"
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/ginkgo/extensions/table"
 	. "github.com/onsi/gomega"