WEB_CONCURRENCY is miscalculated on ECS Fargate

Splitting this out from #210... When running on [Fargate](https://aws.amazon.com/fargate/) with 512MB of RAM, `WEB_CONCURRENCY` is set to `28`. My expectation is that it would be set to `1` (or not set at all).

It looks like the calculations are taken from the following files. Their values on Fargate are:

```
$ cat /sys/fs/cgroup/memory/memory.limit_in_bytes
9223372036854771712
$ cat /sys/fs/cgroup/memory/memory.max
cat: /sys/fs/cgroup/memory/memory.max: No such file or directory
```

It's not yet clear to me the best way to determine how much RAM is available in a Fargate container, but I have an open issue with AWS support and will update when I hear back.