Add empty NodeBuffer interface

The empty NodeBuffer interface acts as a placeholder if node's type
definitions file isn't referenced. This is to hopefully stop the Travis
CI build error.

node-ip/ip-tests.ts

@@ -15,5 +15,5 @@ ip.loopback("ipv6");
 ip.mask("192.168.0.1", "255.255.255.0");
 ip.not("255.255.255.0");
 ip.or("192.168.0.1", "255.255.255.0");
-var buff = ip.toBuffer(myIP);
+var buff:any = ip.toBuffer(myIP);
 ip.toString(buff);

node-ip/ip.d.ts

@@ -3,6 +3,8 @@
 // Definitions by: Peter Harris <https://github.com/codeanimal>
 // Definitions: https://github.com/borisyankov/DefinitelyTyped
 
+interface NodeBuffer { }
+
 declare module "ip" {
     /**
      * Check two IP address are the same.