Chore: Update gitignore to latest version
Chore: Upgrade to Ruby 2.1.2
Feature: Allow reference number length to be customizable

Previously all reference numbers were forced to be 32 hex characters.  We had
a need for the exact functionality that referehencible provides, but for an
attribute where we only wanted 8 characters.

Now we can do that by passing a `length` option to `referenced_by` like so:

```ruby
referenced_by :guid => { length: 8 }
```

You can also mix and match.  If you have some reference attributes which you
want to remain the default length of 32, you can do so like this:

```ruby
referenced_by :guid,
              :access_token,
              :short_code => { length: 8 }
```
Version Bump to 0.4.0
Chore: Upgrade RSpec and RSpectacular