travis: Switch to the Ubuntu 18.04 (bionic) image for tests

Do two full build tests on 16.04 (xenial) and the two for OpenSSL 1.0
also run there.  Since 18.04 ships OpenSSL 1.1.1, which conflicts with
our custom built version, we skip that until OpenSSL 3.0 is released.
A workaround is required for an issue with sonarqube on bionic.

.travis.yml

@@ -1,7 +1,7 @@
 language: c
 
 sudo: required
-dist: xenial
+dist: bionic
 
 # don't build tags separately
 if: tag IS blank
@@ -50,6 +50,11 @@ matrix:
     - env: TEST=all
     - env: TEST=all
       compiler: clang
+    - env: TEST=all
+      dist: xenial
+    - env: TEST=all
+      dist: xenial
+      compiler: clang
     - env: TEST=all MONOLITHIC=yes
     - env: TEST=all MONOLITHIC=yes
       compiler: clang
@@ -86,7 +91,9 @@ matrix:
     - env: TEST=openssl
     - env: TEST=openssl LEAK_DETECTIVE=yes
     - env: TEST=openssl-1.0
+      dist: xenial
     - env: TEST=openssl-1.0 LEAK_DETECTIVE=yes
+      dist: xenial
     - env: TEST=gcrypt
     - env: TEST=gcrypt LEAK_DETECTIVE=yes
     - env: TEST=apidoc