ci: update Travis configuration

 * Drop all versions of gcc < 7 and clang < 5
 * Add clang 11 (development branch)
 * Reenable ASan on Linux with clang 7, the upstream bug has been fixed
 * Drop Xcode 9.2 (macOS 10.12 is no longer supported)
 * Upgrade Xcode 11 to 11.3
 * Workaround issue with missing /usr/local/opt/openssl on macOS
 * Various cleanups

Change-Id: Iee54c0d0480b6ff15bb112a49b3a5c49586ac976
diff --git a/.jenkins b/.jenkins
index 78e1ca1..ea8834d 100755
--- a/.jenkins
+++ b/.jenkins
@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
 set -e
 
+export WAF_JOBS=${WAF_JOBS:-1}
+
 nanos() {
     # Cannot use date(1) because macOS does not support %N format specifier
     python -c 'import time; print(int(time.time() * 1e9))'