Add the version number in the release note page.

1) This will simplify the life of some our users, sysadmins and developers:
https://bugzilla.mozilla.org/show_bug.cgi?id=973335
2) We are already showing them here: https://www.mozilla.org/en-US/firefox/releases/
3) We are using the numbers everywhere, the press is doing the same.
4) The version is available in the URL
Merge pull request #2212 from sylvestre/master

Add the version number in the release note page.