FBR - Upgrade MBS

[Ralph Bean]

I'd like to upgrade MBS on either Monday or Tuesday of next week to
module-build-service-2.6.0-1.el7 and libmodulemd-1.6.3-2.el7.

- https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2018-0a7da7520d
- https://bodhi.fedoraproject.org/updates/FEDORA-EPEL-2018-ee8aeba3a6

There is no database migration.  It has a few bugfixes and a few new features:

- MBS can now block the building of a module stream, if its stream is
  marked as EOL in PDC (requested by mboddu).
- There's also an important bugfix for sgallagh that allows building
  modules that refer to refs of components not under their master
  branch - this is related to the libmodulemd update.

I haven't upgraded staging yet, but would start to do so next week.

+1 to proceed?  Or hold off until after freeze?

-Ralph