Update TypeScript definition to support v6.X

[thinkingserious]

Issue Summary

v6.X is a major breaking change from v5.X, so the TypeScript definition needs an update.

[thinkingserious]

cc @spartan563

[notheotherben]

Will take a look, can't promise that I'll get to it this week with SRECon, however I'll do what I can.

[thinkingserious]

No worries @spartan563, enjoy the conference!

[bogacg]

Meanwhile SPARTAN563's PR gets reviewed, we can trick things a bit to work with Typescript:

// SINCE WE DONT HAVE TYPINGS READY FOR '@sendgrid/mail'
// REQUIRE w. any WILL  BE USED

/// <amd-dependency path="model/tree" />
declare var require: (moduleId: string) => any;
var sgMail = require('@sendgrid/mail');

[thinkingserious]

Thanks @bogacg!

[thinkingserious]

Solved with #430