Add support for Debian 9 (stretch)

Closes #8
diff --git a/Makefile b/Makefile
index 686b81f..24505e4 100644
--- a/Makefile
+++ b/Makefile
@@ -23,6 +23,9 @@
 # Debian 8
 jessie: debian
 
+# Debian 9
+stretch: debian
+
 # Ubuntu 12.04
 precise: find-couch-dist copy-debian precise-prep dpkg lintian