Update Swift 5 version for CI (#67)

Author: Lukasa

docker/docker-compose.1804.50.yaml

@@ -8,7 +8,7 @@ services:
       args:
         ubuntu_version: "18.04"
         swift_version: "5.0"
-        swift_flavour: "DEVELOPMENT-SNAPSHOT-2019-01-23-a"
+        swift_flavour: "DEVELOPMENT-SNAPSHOT-2019-02-19-a"
         swift_builds_suffix: "branch"
         skip_ruby_from_ppa: "true"