[tlu] beta with texdist switching

[Bruno Voisin]

It seems after switching distribution you need to restart TLU to have this taken into account for package updates. I switched from TL-2014 to TL-2015, asked for a package list refresh (with Mojca's pretest mirror as repository), and end up getting

The repository at http://ctan.ijs.si/mirror/tlpretest has TeX Live 2015, but you have TeX Live 2014 installed.  You need to switch repositories in order to continue.

Quitting and restarting TLU, everything's fine (except for a texlive.infra problem, reported separately to the MacTeX list).

Bruno