Add "Find installations" wrapper to Integrations

[offsky]

There is an API call you can make to fetch your installations.

GET /integration/installations
Referenced here: https://developer.github.com/v3/integrations/

This can easily be added to Api/Integrations.php by adding these lines:

public function findInstallations()
    {
        return $this->get('/integration/installations');
    }

I realize that this can be accomplished using the generic $github->getHttpClient()->get('integration/installations'), but this convenience method would be nice.

[m1guelpf]

@offsky Why don't you make a PR?

[Nyholm]

I would be happy to see a PR that fixes this.

[offsky]

I am sorry, but I dont have time right now to make a PR, but I'll put it on my to-do list.

[acrobat]

I've added this method in #527