travis: use Fedora 27 as a base image

It was confirmed experimentally that Fedora 27 is more suitable
for running cov-build than Fedora 28:
https://github.com/systemd/systemd/issues/9186#issuecomment-394577877.

diff --git a/travis-ci/Dockerfile b/travis-ci/Dockerfile
index 9554fcf..71dd07a 100644 (file)
--- a/travis-ci/Dockerfile
+++ b/travis-ci/Dockerfile
@@ -1,8 +1,7 @@
 ## Create Dockerfile that builds container suitable for systemd build
 ## This container runs as non-root user by deafult
 
-# Use the latest stable version of fedora
-FROM fedora:latest
+FROM fedora:27
 
 # Demand the specification of non-root username
 ARG DOCKER_USER