Be robust to upstream changes when building old revs

When running builds that are not from scratch, it is possible that
upstream changes have caused old revisions on master to stop building.
Because homu maintains the invariant that each merge to master is
passing, and we are only building the old revision to create a baseline
to test the new revision, it's safe to ignore failures in the old rev.

.travis/dispatch.sh

@@ -50,7 +50,11 @@ else
     else
         git fetch origin master:master
         git checkout master
+        # Upstream changes could cause the old rev to fail, so disable errexit
+        # (homu will maintain the invariant that each rev on master is passing)
+        set +o errexit
         run_salt 'old'
+        set -o errexit
 
         git checkout "${TRAVIS_COMMIT}"
         run_salt 'upgrade'