Bug 1065659 - Add mozilla-b2g28_v1_3t, staging-gaia-try, mozilla-esr31
Bug 1065659 - Fix update_repository_version to ignore non-active repos

This fixes test failures that occur when there are non-active repos in
the repositories fixture & adds an explicit null-check and raise, rather
than relying on us hitting a TypeError later on.
Bug 1065659 - Hide some obsolete repos to match TBPL

Until we have repo deletion figured out, set the following repos as
onhold, so they are hidden in the UI:
mozilla-esr17
mozilla-b2g18
mozilla-b2g18_v1_1_0_hd
mozilla-b2g26_v1_2
Merge pull request #216 from mozilla/repo-sync

Bug 1065659 - Sync repo list with TBPL