`meteor run` doesn't always write changes to `.meteor/versions`: it only
does so if its release (or checkout-ness) matches `.meteor/release`. So
it preferred to just remember the value of `.meteor/versions` from
rebuild to rebuild rather than forgetting what it knew and re-reading
the possibly-not-updated file.
However, if some other process changes `.meteor/versions`, it would
ignore that change. With this fix, if `.meteor/versions` changes then
that is considered to be the previous versions list, not the last
version list from the same process. For example, this would commonly
happen due to using `meteor update` to update packages (without changing
the tool, which would cause the runner to stop).
Fixes#3582.
a1e002a250