Set Swift version to 4.0.3 in Docker configuration (#89)
Set Swift version to 4.0.3 in Docker configuration (#89)

Motivation:
In order to target the latest version of Swift and address this issue https://github.com/apple/swift-nio/issues/83

Modifications:
Set the Swift version to the latest (4.0.3)

Result:
Uniform version of Swift across the Ubuntu Docker image and a passing
test suite on all target platforms
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio