switch travis distro to bionic

.travis.yml

@@ -1,4 +1,5 @@
 sudo: required
+dist: bionic
 language: node_js
 node_js:
   - "lts/*"
@@ -7,5 +8,5 @@ services:
 script:
   - npm test
 before_install:
-  - docker pull mysql:5.6
+  - docker pull mysql:8
   - docker images

test/docker-compose-testbed.yaml

@@ -9,13 +9,13 @@ networks:
 
 services:
   mysql:
-    image: mysql:5.6
+    image: mysql:8
     environment: 
       MYSQL_RANDOM_ROOT_PASSWORD: "yes"
       MYSQL_DATABASE: jambones
       MYSQL_USER: jambones 
       MYSQL_PASSWORD: jambones
-    command:  --default-authentication-plugin=mysql_native_password
+    command:  --default-authentication-plugin=mysql_native_password 
     ports:
       - "3406:3306/tcp"
     volumes: