Merge branch 'simplify_dockerbuild' of https://github.com/pratikmallya/shellcheck into pratikmallya-simplify_dockerbuild

Move library into src/

Use multi stage Dockerfile to greatly simplify build.

custom-setup:

- http://cabal.readthedocs.io/en/latest/developing-packages.html#custom-setup-scripts
- https://www.well-typed.com/blog/2015/07/cabal-setup-deps/

Bounds:
- containers-0.5 is required if Data.Map.Strict
- parsec-3.0 for Text.Parsec
- json-0.3.6 for makeObj

Check for calls to `which`.

MacOS support for nextnumber

This allows you to use the homebrew install Bash 4 on MacOS systems. It
should compatible with most if not all modern Linux distros.

Favour the builtin `command -v` instead.

[Docs] Update Travis CI part in README.md

Add test for `local` keyword in SC2154

Follow-up to koalaman/shellcheck#988

SC2029: Skip when there are options to ssh

Fixes #327

SC2029 generates false positives when given an ssh command that includes
options with arguments because it assumes the first non-option must be
the host:port argument and the last argument is a command to run.
As suggested the comments on #327, this change fixes those by skipping
the check when there are any options present.

This change adds a test case for a valid command that gets falsely
flagged with SC2029.

Use readVariableName combinator