Fixes a problem with UnreachableStrategy being available now in two
type-incompatible formats (object and string).
We also upgrade the transitive dependency
github.com/donovanhide/eventsource.
Our vendored copy contains a bug that causes unavailable Marathon nodes
to never be marked as available again due to a misconstruction in the
URL to the Marathon health check / ping endpoint used by go-marathon
internally.
A fix[1] has been published.
[1]https://github.com/gambol99/go-marathon/pull/283
