Wrong shellcheck URL in Travis CI

[Sidd-Dino]

Issue: koalaman/shellcheck#1871

shellcheck fails when downloaded from storage.googleapis.com

Failure message:

You are downloading ShellCheck from the wrong URL!

Please update to the latest one:

https://github.com/koalaman/shellcheck/releases/download/latest/shellcheck-latest.linux.x86_64.tar.xz

For more information, see:

https://github.com/koalaman/shellcheck/issues/1871

[Sidd-Dino]

This is the reason why my pull request #147 failed to build

[dylanaraps]

Fixed