attempted fix for infinite invalidation loop

@DerMambo, @tmeasday: can you see if this fixes the issue for you? and
doesn't cause side effects that I'm not thinking of. Use the devel
branch of iron-router. Thanks!

See issues:
* https://github.com/EventedMind/iron-router/issues/742.
* https://github.com/EventedMind/iron-router/issues/741
bump version for bug fix release