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
 * Output folding markers to improve readability of CI logs

Change-Id: I7e6105f100110c74d6e947612fd3df81f7cbf43c
diff --git a/.jenkins.d/00-deps.sh b/.jenkins.d/00-deps.sh
index 5b30418..660fad6 100755
--- a/.jenkins.d/00-deps.sh
+++ b/.jenkins.d/00-deps.sh
@@ -10,7 +10,7 @@
     FORMULAE=(boost openssl pkg-config)
     brew update
     if [[ -n $TRAVIS ]]; then
-        # travis images come with a large number of brew packages
+        # Travis images come with a large number of brew packages
         # pre-installed, don't waste time upgrading all of them
         for FORMULA in "${FORMULAE[@]}"; do
             brew outdated $FORMULA || brew upgrade $FORMULA
@@ -32,7 +32,7 @@
     fi
 fi
 
-if has CentOS $NODE_LABELS; then
+if has CentOS-7 $NODE_LABELS; then
     sudo yum -y install yum-utils pkgconfig \
                         openssl-devel libtranslit-icu \
                         python-devel sqlite-devel \