updated to new (better) sendgrid api location

We (SendGrid) [recently opened up](http://support.sendgrid.com/hc/en-us/articles/200738596-New-Web-API-URL-api-sendgrid-com) `api.sendgrid.com/api/` as a new endpoint for our API.

It has a number of benefits over the old `sendgrid.com/api/`, including the possibility of faster connection times.