- update contrib to 034a156 and enable mod_http_upload

- disable mod_multicast from the contrib repository
a) it already is part of core it seems
b) it installs its own ejabberd_c2s.beam which does not match
the new code in 15.06

- update to 15.06 and revision 2a953da of ejabberd-contrib:
Important update note:
mod_mam has now 2 implementations: mod_mam and mod_mam_mnesia
The later is the original mod_mam for ejabberd from the contrib
repository. The mod_mam that now comes with ejabberd core has
nothing in common with the old implementation and also does not
migrate data from mod_mam_mnesia. If you have been using the old
mod_mam, you might want to switch to mod_mam_mnesia unless you
need the SQL features that come with the new implementation
This means you might want to change your config to load
mod_mam_mnesia *before* updating the package.