remove 'lint' target from travis CI builds

the shellcheck developer unilaterally decided removal of zsh support
https://github.com/koalaman/shellcheck/issues/298 so far ignoring all
requests to add support, so we won't run our shellcheck tests on
travis. using Devuan Jessie's 0.3.5 with our ignores still works, so
will keep testing it locally.

.travis.yml

@@ -14,12 +14,10 @@ addons:
       - steghide
       - qrencode
       - e2fsprogs
-      - shellcheck
 
 before_script:
   - make --directory=extras/kdf-keys
   - sudo make --directory=extras/kdf-keys install
 
 script:
-  - make lint
   - make test