updated meson test

hopefully, bionic will have a more recent version of python
required to install meson.

.travis.yml

@@ -124,8 +124,8 @@ matrix:
 
     # meson dedicated test
     - name: Xenial (Meson + clang)    # ~15mn
-      if: branch = release
-      dist: xenial
+      if: branch = travisTest
+      dist: bionic
       language: cpp
       compiler: clang
       install: