Update eslint requirement from ^4.0.0 to ^5.15.0

Updates the requirements on [eslint](https://github.com/eslint/eslint) to permit the latest version.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](eslint/eslint@v4.0.0...v5.15.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

package.json

@@ -21,7 +21,7 @@
   "devDependencies": {
     "chai": "^4.0.2",
     "coveralls": "^3.0.0",
-    "eslint": "^4.0.0",
+    "eslint": "^5.15.0",
     "istanbul": "^1.1.0-alpha.1",
     "mocha": "^3.4.2",
     "sinon": "^2.3.4"