CircleCI: build on xenial instead of trusty

MCMrARM · MCMrARM · commit 5b899a6ebfb3 · 2018-09-17T19:27:54.000+02:00
diff --git a/.circleci/config.yml b/.circleci/config.yml
@@ -2,7 +2,7 @@ version: 2
 jobs:
   build:
     docker:
-    - image: buildpack-deps:trusty
+    - image: buildpack-deps:xenial
     steps:
     - checkout
     - run:
