Update to latest dev snapshot at time of this commit (2019-08-24)

Author: gwynne

5.1/ubuntu/18.04/Dockerfile

@@ -28,7 +28,7 @@ RUN export DEBIAN_FRONTEND=noninteractive DEBCONF_NONINTERACTIVE_SEEN=true && ap
 ARG SWIFT_SIGNING_KEY=8513444E2DA36B7C1659AF4D7638F1FB2B2B08C4
 ARG SWIFT_PLATFORM=ubuntu18.04
 ARG SWIFT_BRANCH=swift-5.1-branch
-ARG SWIFT_VERSION=swift-5.1-DEVELOPMENT-SNAPSHOT-2019-06-21-a
+ARG SWIFT_VERSION=swift-5.1-DEVELOPMENT-SNAPSHOT-2019-08-24-a
 
 ENV SWIFT_SIGNING_KEY=$SWIFT_SIGNING_KEY \
     SWIFT_PLATFORM=$SWIFT_PLATFORM \