Switch to bionic (Ubuntu 18.04) due to jdk8

VolkerHartmann · VolkerHartmann · commit 8d580c6e2178 · 2020-12-16T07:19:47.000+01:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,6 @@
 language: java
 
-dist: focal
+dist: bionic
 
 jdk:
 # Enable all versions which should be supported
