use bionic for almost all tasks for now in travis (#2685)

diff --git a/.travis.yml b/.travis.yml
index 7cd5b2d..41a8a3f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -47,7 +47,7 @@
     - SCRIPT="cmake.sh"
     - BUILD_ARG=""
     - BUILD_ENV="-e CC=gcc -e CXX=g++ -e THRIFT_CROSSTEST_CONCURRENCY=4"
-    - DISTRO=ubuntu-focal
+    - DISTRO=ubuntu-bionic
     - BUILD_LIBS="CPP C_GLIB JAVA PYTHON TESTING TUTORIALS"  # only meaningful for CMake builds
     - TRAVIS_BUILD_STAGE=test
     # DOCKER_REPO (this works for all builds as a source for docker images - you can override for fork builds in your Travis settings)