Test against ESLint 5

[edmorley]

Hi!

Currently the Travis configuration for this repo does:

env:
  - ESLINT_VERSION=latest
  - ESLINT_VERSION=3.15.0
before_script:
  - if [[ $ESLINT_VERSION != "latest" ]]; then
      yarn upgrade "eslint@$ESLINT_VERSION";
fi

And in package.json:

  "devDependencies": {
    "eslint": "^3.14.1",

At first glance it might appear that eslint-plugin-prettier is being tested against "latest" (ie 5.x) and 3.15.0. However given the package.json eslint version is out of date, the plugin is actually only being tested against ESLint 3.19.0 and 3.15.0 - meaning there is no test coverage of 4.x or 5.x.

I would suggest either:

1. Enabling something like https://renovatebot.com/ for this repository (so the package.json version is actually "latest"), and renaming "latest" in .travis.yml to something more accurate, like "default".
2. Changing the before_script .travis.yml section to remove the conditional, so that it always runs yarn upgrade "eslint@$ESLINT_VERSION", including in the "latest" case.

I think option (1) seems preferable, since Renovate would also ensure other dependencies are kept up to date -- plus (2) would be confusing, since someone just looking at package.json (and not also .travis.yml) might mistakenly think there was still no test coverage of more recent ESLint versions.