Don't unnecessarily install shellcheck on the lint build

Empact · Empact · commit 515348f7043f · 2018-07-21T12:57:04.000-04:00
"Travis CI has now integrated ShellCheck by default, so you don't need to manually install it." https://github.com/koalaman/shellcheck#travis-ci
diff --git a/.travis.yml b/.travis.yml
@@ -88,7 +88,6 @@ jobs:
         apt:
           packages:
             - python3-pip
-            - shellcheck
       install:
         - travis_retry pip3 install flake8 --user
       before_script:
