documentation: prepare for 3.1.8 release

(From yocto-docs rev: 09b64a4d246bdcca62dddee152deef7b0dea69d7)

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/sphinx-static/switchers.js

@@ -3,7 +3,7 @@
 
   var all_versions = {
     'dev': 'dev (3.3)',
-    '3.1.7': '3.1.7',
+    '3.1.8': '3.1.8',
     '3.0.4': '3.0.4',
     '2.7.4': '2.7.4',
   };