Newer versions of the retryablehttp package use a context, so we need to
add that in our custom `CheckRetry` function.
In addition I removed the `return true, nil` to continue retrying in
case of an error, and instead directly call the `DefaultRetryPolicy`.
This is because the `DefaultRetryPolicy` will now also take the context
into consideration.
We've missed a few recent additions to go.mod in the vendor directory. We
need to keep this updated for the moment until all of the surrounding
tooling is ready to go all-in with Go 1.11 modules.
This work was done against APIs that were already changed in the branch
before work began, and so it doesn't apply to the v0.12 development work.
To allow v0.12 to merge down to master, we'll revert this work out for now
and then re-introduce equivalent functionality in later commits that works
against the new APIs.
Pam Selle