Update git submodules

* Update ChronoSync from branch 'master'
  to dfd704aa4e9c372b008275a7bae0422336ba4d1a
  - ci: upgrade Travis config to Ubuntu 18.04
    
    Also in this commit:
    
     * Drop clang 3.8 (no easy way to install it on 18.04)
     * Add clang 10 (trunk)
     * Drop Xcode 8.3
     * Upgrade Xcode 10.2 to 10.3
     * Add Xcode 11
    
    Change-Id: Ic8fd05fdacdfc190c722244372a8852705762f32
    
diff --git a/ChronoSync b/ChronoSync
index d7de2d4..dfd704a 160000
--- a/ChronoSync
+++ b/ChronoSync
@@ -1 +1 @@
-Subproject commit d7de2d45b2a7c5458a740ebb2a785c994e307349
+Subproject commit dfd704aa4e9c372b008275a7bae0422336ba4d1a