Let's test re-enabling shellcheck in CI

shellcheck is a truly valuable utility. Unfortunately installing the
packages from debian-sid introduces a conflict. Let's try another
suggestion from: https://github.com/koalaman/shellcheck/wiki/TravisCI

Refs
https://github.com/heroku/heroku-buildpack-python/commit/a0275888a26b8f0132c7bb9fe3bf194b5c4fab1f

.travis.yml

@@ -2,15 +2,12 @@ language: bash
 dist: trusty
 jobs:
   include:
-    # - stage: "Bash linting (shellcheck)"
-    #   sudo: false
-    #   addons:
-    #     apt:
-    #       sources:
-    #       - debian-sid    # Grab shellcheck from the Debian repo (o_O)
-    #       packages:
-    #       - shellcheck
-    #   script: make check
+    - stage: "Bash linting (shellcheck)"
+      sudo: false
+      before_install:
+        - wget -c https://goo.gl/ZzKHFv -O - | tar -xvJ -C /tmp/
+        - PATH="/tmp/shellcheck-latest:$PATH"
+      script: make check
 
     - stage: "Stack Tests"
       services: docker