Update HubSpot tracking code

[craigbruce]

The tracking code for HubSpot is out of date. It is now async and only requires your Hub/Portal ID (no domain). I'm proposing I update the code and remove the HUBSPOT_DOMAIN variable.

You can see snippet of the new code at the base of http://knowledge.hubspot.com/getting-started-with-hubspot/how-to-install-the-tracking-code-on-a-generic-website

Any objections to writing a PR for this?

[jcassee]

Hi Craig,

Absolutely not, please create a PR and I'll merge in the required changes. Thanks for picking this up.

Joost

[jcassee]

Fixed in #39. Thanks Craig!