refactor: replace deprecated String.prototype.substr() (#196)

index.js

@@ -270,7 +270,7 @@ RedirectableRequest.prototype._performRequest = function () {
   // If specified, use the agent corresponding to the protocol
   // (HTTP and HTTPS use different types of agents)
   if (this._options.agents) {
-    var scheme = protocol.substr(0, protocol.length - 1);
+    var scheme = protocol.slice(0, -1);
     this._options.agent = this._options.agents[scheme];
   }