[eslint] Enforce rule 18.5

Rule 18.5 [1] states that files shall end with a single newline
character. Until now, this was not checked because there was no such
option in eslint.

Since version 1.8.0, eslint provides the ability to do that. See pull
request on eslint [2] for details on implementation and usage. Let's use
it in airbnb eslint plugin now!

[1]: https://github.com/airbnb/javascript#18.5
[2]: https://github.com/eslint/eslint/pull/4266

packages/eslint-config-airbnb/rules/style.js

@@ -50,8 +50,8 @@ module.exports = {
     'no-lonely-if': 0,
     // disallow mixed spaces and tabs for indentation
     'no-mixed-spaces-and-tabs': 2,
-    // disallow multiple empty lines
-    'no-multiple-empty-lines': [2, {'max': 2}],
+    // disallow multiple empty lines and only one newline at the end
+    'no-multiple-empty-lines': [2, {'max': 2, 'maxEOF': 1}],
     // disallow nested ternary expressions
     'no-nested-ternary': 2,
     // disallow use of the Object constructor