Fix implementation on s390x

[npmccallum]

https://travis-ci.org/enarx/frenetic

At the above link you can see that tests fail for s390x. At present, curl crashes. When I downgraded to debian:stretch then curl succeeded but the resulting Rust binary for s390x installation crashes. I gave up at that point.

In order to solve this bug you need to:

1. Remove s390x from allow_failures.
2. Make all required tests pass.
3. Submit a pull request.