Add symlink to sid template where this module is looking for it (#12).

Fixes this regression brought by dropping lsb facts:

  Could not find template 'postfix/master.cf.debian-stretch/sid.erb'

diff --git a/templates/master.cf.debian-stretch/sid.erb b/templates/master.cf.debian-stretch/sid.erb
new file mode 120000 (symlink)
index 0000000..e861e80
--- /dev/null
+++ b/templates/master.cf.debian-stretch/sid.erb
@@ -0,0 +1 @@
+../master.cf.debian-sid.erb
\ No newline at end of file