[rb] specify geckodriver 0.15 is required [ci skip]

lmtierney · lmtierney · commit 7a15be8742f1 · 2017-03-08T10:23:10.000-06:00
diff --git a/rb/CHANGES b/rb/CHANGES
@@ -1,6 +1,9 @@
 3.3.0 (2017-03-07)
 ===================
 
+Firefox:
+  * geckodriver v0.15 or later is required
+
 W3C:
   * Support for command response data to be wrapped in a 'value' key
   * Support for updated timeout formats
