[Tests] switch to use Cabal to install ShellCheck

1. This can help speed up the apt process since we decrease the
complexity of apt sources and package dependencies resolving by dropping
an additional ppa source.

2. Stack doesn't update ShellCheck to v0.4.5 after it has been released
more than one month, Cabal can provide ShellCheck v0.4.5

.travis.yml

@@ -4,29 +4,28 @@ addons:
   apt:
     sources:
       - ubuntu-toolchain-r-test
-      - fpcomplete-precise
       - llvm-toolchain-precise
       - llvm-toolchain-precise-3.8
     packages:
       - gcc-4.8
       - g++-4.8
       - clang-3.8
       - lldb-3.8
-      - stack
-      - bc
+      - cabal-install
+      - ghc
       - zsh
       - ksh
 cache:
   directories:
-    - $HOME/.stack
+    - $HOME/.cabal
     - $TRAVIS_BUILD_DIR/.cache
 before_install:
   - $SHELL --version 2> /dev/null || dpkg -s $SHELL 2> /dev/null || which $SHELL
   - curl --version
   - wget --version
   - clang --version
   - clang++ --version
-  - if [ -n "${SHELLCHECK-}" ]; then stack setup && stack install ShellCheck && shellcheck --version ; fi
+  - if [ -n "${SHELLCHECK-}" ]; then cabal update && cabal install transformers-0.4.3.0 ShellCheck && shellcheck --version ; fi
   - if [ -z "${SHELLCHECK-}" ]; then sudo ln -sf /usr/bin/clang-3.8 /usr/bin/clang && sudo ln -sf /usr/bin/clang++-3.8 /usr/bin/clang++ && clang --version ; fi
 install:
   - (mkdir /tmp/urchin && cd /tmp/urchin && curl -s "$(curl -s https://registry.npmjs.com/urchin | grep -Eo '"tarball":\s*"[^"]+"' | tail -n 1 | awk -F\" '{ print $4 }')" -O && tar -x -f urchin*)
@@ -39,7 +38,7 @@ env:
   global:
     - CXX=g++-4.8
     - CC=gcc-4.8
-    - PATH=$(echo $PATH | sed 's/::/:/')
+    - PATH="~/.cabal/bin/:$(echo $PATH | sed 's/::/:/')"
     - NVM_DIR="${TRAVIS_BUILD_DIR}"
   matrix:
     - SHELLCHECK=true