Downgrade to Bluebird 3.5.5 to fix Firefox
Bluebird 3.6.0+ currently breaks in at least Firefox with errors in their
rejection tracking approach. For now, this retreats back to the version we had
used for a long time.
Regressed by https://github.com/matrix-org/matrix-js-sdk/pull/1053
Fixes https://github.com/vector-im/riot-web/issues/11148
