Fix missing trailing newline in sources.list

Re-implement MR #49 by Diederik de Haas.

raspi_master.yaml

@@ -49,11 +49,11 @@ steps:
     unless: rootfs_unpacked
 
   - create-file: /etc/apt/sources.list
-    trailing-newline: '1'
-    contents: |
+    contents: |+
       deb http://deb.debian.org/debian __RELEASE__ main contrib non-free
       deb http://security.debian.org/debian-security __SECURITY_SUITE__ main contrib non-free
       __BACKPORTS__
+
     unless: rootfs_unpacked
 
   - copy-file: /etc/initramfs-tools/hooks/rpi-resizerootfs