fix for travis failure at b0dda229c9

according to https://github.com/browserstack/browserstack-local-nodejs/issues/19#issuecomment-256040404

lib/github.js

@@ -104,6 +104,7 @@ export class GitHub {
     return new Promise((resolve, reject) => {
       const headers = { Accept: 'application/octet-stream' };
       const ws = fs.createWriteStream(file);
+      let result;
       const req = progress(this.request.get(url, {
         headers
       }, (error, response) => {
@@ -118,7 +119,7 @@ export class GitHub {
         }
         log.showProgress(100);
         log.disableProgress();
-        resolve(response);
+        result = response;
       }));
       req.on('progress', (state) => {
         let p;
@@ -130,6 +131,11 @@ export class GitHub {
         log.showProgress(p * 100);
       });
       req.pipe(ws);
+      ws.on('close', () => {
+        resolve(result);
+      }).on('error', (error) => {
+        reject(error);
+      });
     });
   }