use the Swift 5.0 snapshot from 2018-12-28 (#724)
use the Swift 5.0 snapshot from 2018-12-28 (#724)

Motivation:

that snapshot fixes https://bugs.swift.org/browse/SR-9518 which blocks
the allocation tests and therefore #710 .

Modifications:

made our docker use the 2012-12-28 snapshot.

Result:

allocation tests hopefully work again because Swift doesn't accidentally
forget to decrease some reference count anymore.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio