Revert "Update selenium from 3.13.0 to 3.14.0"

This reverts commit c157912.

Version 3.14.0, released on 2018-08-02, includes a bug which regresses
support for the "RemoteDriver" [1]. That interface is used in this
project for integration with Apple Safari. The package upgrade was fully
automated and not motivated by any specific bug fix or feature. Revert
the upgrade to restore functionality.

[1] SeleniumHQ/selenium#6250

tools/wptrunner/requirements_safari.txt

@@ -1,2 +1,2 @@
 mozprocess == 0.26
-selenium==3.14.0
+selenium==3.13.0