Fix token expiration

`expires_in` property of returned token represents a number of seconds (valid ~1 year), while current implementation assumes a unix timestamp.
Because of this assumption, [following code](https://github.com/blainsmith/node-automatic/blob/819fc5104b06f9f8c57f64d9897e774cf8f34d8a/lib/automatic.js#L78):
```javascript
return (this.token.expires_in > new Date().getTime()); // expires_in == 31535999
```
will always return false.

This PR:
* add reference to current date before requesting token.
* use this reference when checking if token is valid.
* fix mocked value for expires_in in test.
* add test to make sure the token actually expires.