Official support for RANDOM.ORG API v2

[willfrew]

There's now a second release of the API that includes some additional api methods (see here for docs). We'll need to switch to the new endpoint by default and add support for those new methods.

#8 makes us compatible with the new api at least and for now it's possible to pass { endpoint: 'https://api.random.org/json-rpc/2/invoke' } to the RandomOrg constructor to switch to the new one but this isn't ideal.

[willfrew]

Closing this since 2.0.0 now defaults to v2 of the API.
I'll create a separate issue for support of the new methods.