geckodriver: changes: v0.16.0 has not been released

Prevent confusion amongst users as towards whether v0.16.0 is out.

Source-Repo: https://github.com/mozilla/geckodriver
Source-Revision: dccea97b6cc709b4bd93146ea12c7d9b0b356688

--HG--
extra : rebase_source : 19985f1ae4f75089c1ee26e5d896292d82a82b0c

testing/geckodriver/CHANGES.md

@@ -2,7 +2,7 @@
 
 All notable changes to this program is documented in this file.
 
-## 0.16.0 (2017-04-10)
+## Unreleased
 
 ### Added
 - Support for WebDriver-conforming [New Session](https://w3c.github.io/webdriver/webdriver-spec.html#dfn-new-session) negotiation, with `desiredCapabilities`/`requiredCapabilities` negotiation as fallback