Fix ".travis.yml" handling of "$files" :innocent:

.travis.yml

@@ -13,7 +13,7 @@ script:
       echo >&2 "Need markdownfmt:";
       echo >&2 "$files";
       echo >&2;
-      markdownfmt -d  "$files" >&2;
+      echo "$files" | xargs markdownfmt -d >&2;
       exit 1;
     fi
   - failed='';