Add 7.13 changelog section.
Add 7.13 changelog section.

aleksandrs-ledovskis/omnibus-gitlab,felixbuenemann/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,aleksandrs-ledovskis/omnibus-gitlab,felixbuenemann/omnibus-gitlab,gitlabhq/omnibus-gitlab,felixbuenemann/omnibus-gitlab,copystudy/omnibus-gitlab,copystudy/omnibus-gitlab,copystudy/omnibus-gitlab,aleksandrs-ledovskis/omnibus-gitlab