Use ubuntu bionic/18.04 to build
Use ubuntu bionic/18.04 to build

By default travis uses 16.04 with docker 18.06, which is quite old...
liimaorg/liima,liimaorg/liima,liimaorg/liima,liimaorg/liima,liimaorg/liima,liimaorg/liima,liimaorg/liima,liimaorg/liima