tldjs gives inconsistent results

Consider the following test case:

```
$ node
> var tld = require('tldjs');
> tld.getSubdomain('www.majestic12.co.uk');
'www'
> tld.getSubdomain('www.bl.uk');
'www'
> tld.getSubdomain('www.majestic12.co.uk');
'www.majestic12'
```

Notice the first and the third run, where getSubdomain() is given the exact same input.

Expectation: Same result from tld.getSubdomain(), when the function is given same input.

Actual: Different result from tld.getSubdomain(), dependent on what other domains have been parsed in the mean time between the two calls.