restore old travis conf

.travis.yml

@@ -1,10 +1,16 @@
 language: minimal
 
-dist: focal
+dist: bionic
 
 services:
 - docker
 
+# docker >= 19.03 is required for docker buildx, so it is upgraded
+before_install:
+- sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
+- sudo apt update -y
+- sudo apt install --only-upgrade docker-ce -y
+
 script:
 - make test