test: install shellcheck and lint Shell scripts

Author: bdossantos

.travis.yml

@@ -1,10 +1,19 @@
 language: bash
 before_install:
   - git clone --depth 1 https://github.com/sstephenson/bats.git
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update; brew install gnu-sed; fi
-
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
+      brew update;
+      brew install gnu-sed;
+      brew install shellcheck;
+    fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
+      sudo add-apt-repository "deb http://archive.ubuntu.com/ubuntu/ trusty-backports restricted main universe";
+      sudo apt-get -y update;
+      sudo apt-get -y install shellcheck;
+    fi
 script:
   - ./bats/bin/bats test/*.bats
+  - shellcheck diff-so-fancy update-deps.sh
 os:
   - linux
   - osx