Restore Linux compatibility

A previous commit attempted to use a sed invocation that would work on
OSX and Linux. But it doesn't. The wisdom of StackOverflow is that
there is no such invocation if you want to use in-place editing. We
can work around this somehow by not doing in-place, but I'm going to
leave that until someone actually needs to run this on OSX again.

test/helpers.sh

@@ -15,8 +15,8 @@ docker_run() {
 
 docker_compose_up() {
   local l_image="$1" l_cname="$2" l_composefile="$3"; shift; shift; shift
-  sed -i '' -e "s|image: .*|image: ${l_image}|g" "${l_composefile}"
-  sed -i '' -e "s|container_name: .*|container_name: ${l_cname}|g" "${l_composefile}"
+  sed --in-place -e "s|image: .*|image: ${l_image}|g" "${l_composefile}"
+  sed --in-place -e "s|container_name: .*|container_name: ${l_cname}|g" "${l_composefile}"
 
   docker-compose --file "${l_composefile}" --project-name test up -d
   trap "docker-compose --file ${l_composefile} down" EXIT