Make travis run shellcheck only on shellscripts

There are only a few scripts in bin/, but not all might be shell. At
least for the moment, it seems reasonable to explicitely enumerate all
shell-scripts in bin/ for shellcheck …

Signed-off-by: Dario Ernst <dario@kanojo.de>

Author: Dario Ernst

.travis.yml

@@ -22,7 +22,7 @@ jobs:
         - export PATH="$HOME/.yarn/bin:$PATH"
     - env: task=ShellCheck
       script:
-        - shellcheck bin/*
+        - shellcheck bin/heroku bin/setup
       language: generic
     - env: task=doctoc
       install: npm install doctoc