Update to 31.6.0

CHangelog:
Fixed in Thunderbird 31.6

    2015-40 Same-origin bypass through anchor navigation
    2015-37 CORS requests should not follow 30x redirections after preflight
    2015-33 resource:// documents can load privileged pages
    2015-31 Use-after-free when using the Fluendo MP3 GStreamer plugin
    2015-30 Miscellaneous memory safety hazards (rv:37.0 / rv:31.6)

(ryoon)