Fix assertion config error - Gonzales-3.2

[DanPurdy]

Fixes #563

This is one I've sort of known about for a long time but finally got round to investigating it a bit.

Currently if you're using the CLI and you don't supply a path a config file or have a config file in the root of your project then the config loader throws an error to say it can't find a path. This was mainly down to the way the config loader would fall back to your package.json if it couldn't find a .sass-lint.yml file, the logic wasn't taking into account a scenario where that path wasn't specified in your package file.

This resolves it and adds a test to ensure this error doesn't reoccur.

I've pointed it towards the gonzales 3.2 update branch as I don't want to have to keep updating that one with new develop work, we may as well just load it all together in preparation for 1.6.

<DCO 1.1 Signed-off-by: Dan Purdy danjpurdy@gmail.com>

[coveralls]

Coverage decreased (-0.1%) to 96.961% when pulling d920464 on DanPurdy:feature/fix-assertion-config-error into 4210db7 on sasstools:feature/gonzales-3-develop.

[bgriffith]

Looks good.