CI: update to Swift snapshot 13th Dec 2018 (#703)
CI: update to Swift snapshot 13th Dec 2018 (#703)

Motivation:

SwiftPM was very outdated on the old Swift snapshots, we should update
the CI.

Modifications:

use swift-5.0-DEVELOPMENT-SNAPSHOT-2018-12-13-a-ubuntu18.04.tar.gz

Result:

can use swift-tools-version: 5.0 and all the goodies from that
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio