Use a different ubuntu image for CircleCI

To avoid the recent issues with apt locks, we are moving to a more
recent release.

Author: equalsraf

.circleci/config.yml

@@ -2,7 +2,7 @@ version: 2
 jobs:
   build:
     machine:
-      image: circleci/classic:201808-01
+      image: ubuntu-1604:201903-01
     working_directory: ~/project
     steps:
       - checkout