Update GoSquared tracker to use latest tracker functions

Everything seems to work, however the tests for identify do not pass. Looks like it's because `test(gosquared`) overwrites the already loaded tracking code?? and the `_gs('get', 'blah')` functionality requires the tracker to have loaded to work.

```js
// line 116 of test/integrations/gosquared
console.log(window._gs); // ==> fully loaded function
```

```js
// line 129 of lib/gosquared
console.log(window._gs); // ==> placeholder function
```