get shellcheck from stretch

sid has a version that is incompatible with travis' ubuntu 14.04

.travis.yml

@@ -1,12 +1,14 @@
 language: bash
 
 # Use container-based infrastructure for quicker build start-up
-sudo: false
+sudo: required
 
 addons:
   apt:
     sources:
-    - debian-sid    # Grab shellcheck from the Debian repo (o_O)
+    # Grab shellcheck from the Debian repo (o_O)
+    - sourceline: deb http://ftp.us.debian.org/debian stretch main contrib non-free
+      key_url: https://ftp-master.debian.org/keys/archive-key-9.asc
     packages:
     - shellcheck