zlib deprecation prevention (#119)

Access Z_SYNC_FLUSH is still possible but deprecated. Prevented this deprecation by accessing Z_SYNC_FLUSH from constants namespace.

Author: Leafgard

README.md

@@ -18,10 +18,10 @@ app.use(compress({
   },
   threshold: 2048,
   gzip: {
-    flush: require('zlib').Z_SYNC_FLUSH
+    flush: require('zlib').constants.Z_SYNC_FLUSH
   },
   deflate: {
-    flush: require('zlib').Z_SYNC_FLUSH,
+    flush: require('zlib').constants.Z_SYNC_FLUSH,
   },
   br: false // disable brotli
 }))