Attempt to fix the build

.travis.yml

@@ -31,13 +31,13 @@ matrix:
         - GENERATOR="Visual Studio 15 2017 Win64" BUILD_TYPE=Debug EXTRA_CHECKS=ON TRACE=ON TOOLS=ON TESTS=ON
         - GENERATOR="Visual Studio 15 2017 Win64" BUILD_TYPE=Release EXTRA_CHECKS=OFF TRACE=OFF TOOLS=ON TESTS=ON
     - os: linux
-      dist: xenial
+      dist: bionic
       compiler: gcc
       env:
-        - GENERATOR="Unix Makefiles" BUILD_TYPE=Debug EXTRA_CHECKS=ON TRACE=ON TOOLS=ON TESTS=ON CC=gcc-6 CXX=g++-6
+        - GENERATOR="Unix Makefiles" BUILD_TYPE=Debug EXTRA_CHECKS=ON TRACE=ON TOOLS=ON TESTS=ON
         - GENERATOR="Unix Makefiles" BUILD_TYPE=Release EXTRA_CHECKS=OFF TRACE=OFF TOOLS=ON TESTS=ON
     - os: linux
-      dist: xenial
+      dist: bionic
       compiler: clang
       env:
         - GENERATOR="Unix Makefiles" BUILD_TYPE=Debug EXTRA_CHECKS=ON TRACE=ON TOOLS=ON TESTS=ON