Adding jest matchers to conveniently use targaryen with jest.

Usage:
```
const { getDatabase, toAllowRead, toAllowUpdate, toAllowWrite, json } = require('targaryen/plugins/jest');

expect.extend({
  toAllowRead,
  toAllowUpdate,
  toAllowWrite,
});

const RULES_PATH = 'database.rules.json';
const rules = json.loadSync(RULES_PATH);
const initialData = {};

const database = getDatabase(rules, initialData);

test('basic', () => {
  expect(database.as(null)).not.toAllowRead('/user');
  expect(database.as(null)).toAllowRead('/public');
  expect(database.as({ uid: '1234'})).toAllowWrite('/user/1234', {
    name: 'Anna',
  });
});

```

or using the generic matcher:
```
const { getDebugDatabase, toBeAllowed, json } = require('targaryen/plugins/jest');

expect.extend({
  toBeAllowed,
});

const RULES_PATH = 'database.rules.json';
const rules = json.loadSync(RULES_PATH);
const initialData = {};

// NOTE: Create a database with debug set to true for detailed errors
const database = getDebugDatabase(rules, initialData);

test('generic', () => {
  expect(database.as(null).read('/user')).not.toBeAllowed();
  expect(database.as({ uid: '1234' }).write('/user/1234', {
    name: 'Anna',
  })).toBeAllowed();
});

```

As jest requires node v6+ don't run tests if a version lower than 6 is
detected.

Using grep is less future-proof but the better option, as comparing
using `<` can be incorrect.