Add import plugin and test environment settings

Add eslint-plugin-import
> https://github.com/benmosher/eslint-plugin-import#readme

Add test env jest / mocha
> Included in `@fiverr/fiverr`:
> - `base`: Just the set of rules
> - `es6`: New ES support and rules
> - `react`: React specific rules and `eslint-plugin-react` plugin
>
> Need to be added explicitly:
> - `jest`
> - `mocha`

package.json

@@ -1,7 +1,7 @@
 {
   "name": "@fiverr/eslint-config-fiverr",
   "moduleName": "eslint-config-fiverr",
-  "version": "3.0.0",
+  "version": "3.1.0",
   "description": "Fiverr's ESLint config",
   "author": "Fiverr",
   "license": "MIT",