Use linter for shell scripts

[kc1212]

I recommend shellcheck

[ValentinMoullet]

From the ShellCheck Github page: "Travis CI has now integrated ShellCheck by default, so you don't need to manually install it."

But I wasn't sure how we should integrate the linting of Go files with Bash files; do we need another different .travis.yml only for bash files?
Also we need to add it to when we make test.