travis: getting latest shellcheck is hard

Trying instructions from https://github.com/koalaman/shellcheck

Author: docwhat

.travis.yml

@@ -1,10 +1,13 @@
 sudo: false
+language: bash
 
 addons:
   apt:
+    sources:
+      - debian-sid # Grab ShellCheck from the Debian repo
     packages:
-    - shellcheck
-    - bash
+      - bash
+      - shellcheck
 
 script:
   - ./citest