Use prettier-linter-helpers

[BPScott]

eslint-plugin-prettier v3 removed the generateDifferences and showInvisibles functions as they are now provided by the prettier-linter-helpers package. Depend upon that package instead of eslint-plugin-prettier.

Here's the PR in stylelint-prettier that changed from depending on eslint-plugin-prettier to prettier-linter-helpers to demonstrate the changes required.

[ikatyang]

Thanks for the notice. This will be a breaking change (require node 6) but it's actually just an internal change so I'll wait for other breaking changes.