Fix broken gflags link

Summary: Fix broken gflags link

Test Plan: Follow the link

Reviewers: igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D40503

INSTALL.md

@@ -28,7 +28,7 @@ your make commands, like this: `PORTABLE=1 make static_lib`
       data compression.
 
 * All our tools depend on:
-  - [gflags](https://code.google.com/p/gflags/) - a library that handles
+  - [gflags](https://gflags.github.io/gflags/) - a library that handles
       command line flags processing. You can compile rocksdb library even
       if you don't have gflags installed.