Bump to jQuery 1.12.*

[erkyrath]

I don't see any significant road bumps in the changelog:

• https://blog.jquery.com/2016/01/08/jquery-2-2-and-1-12-released/
• https://github.com/jquery/jquery/compare/1.12.0...1.12.4

(This will eventually let us unify the jquery files in Lectrote.)