Fix #97 unref() the rotation timeout in RotatingFileStream.

The rotation `setTimeout` from `RotatingFileStream` will prevent the process from exiting.
This fix simply `unref()` the timeout immediately after it is set.

lib/bunyan.js

@@ -990,6 +990,9 @@ RotatingFileStream.prototype._setupNextRot = function () {
     this.timeout = setTimeout(
         function () { self.rotate(); },
         this.rotAt - Date.now());
+    if (typeof this.timeout.unref === 'function') {
+      this.timeout.unref();
+    }
 }
 
 RotatingFileStream.prototype._nextRotTime = function _nextRotTime(first) {