please upgrade the package for support new send-grid library

[alikomijani]

please upgrade the package for support new send-grid library
currently send-grid release version 6.4.6 but your app only support version 3.6

[wpet]

I strongly support the request of @alikomijani. An upgrade would be very much appreciated.

[petrklus]

PR submitted #95

[mikemanger]

sklarsa/django-sendgrid-v5 seems to be the email backend library Sendgrid recommend currently. As far as I can tell it is a drop in replacement for this one, just change the backend and look at the new config.