travis: add shellcheck call for all scripts in the repository

Scripts are detected by shebang, not an ideal approach, but should be
good enough.

.travis.yml

@@ -18,6 +18,7 @@ jobs:
       env: TESTS_ONLY=1
       script:
        - ./run-tests
+       - shellcheck  $(grep -l '^#!/bin/\(ba\)\?sh' $(git ls-files))
       after_success:
        - codecov
     - stage: deploy