Upgrade to sendgrid 4

[pcraciunoiu]

Tested this on a simple django 1.11 configuration.

See SendGrid changelog:

[codecov-io]

Codecov Report

Merging #55 into master will not change coverage.
The diff coverage is 45.45%.

@@           Coverage Diff           @@
##           master      #55   +/-   ##
=======================================
  Coverage   60.22%   60.22%           
=======================================
  Files           1        1           
  Lines          88       88           
  Branches       22       22           
=======================================
  Hits           53       53           
  Misses         29       29           
  Partials        6        6

Impacted Files	Coverage Δ
sgbackend/mail.py	60.22% <45.45%> (ø)	⬆️

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[pcraciunoiu]

@elbuo8 any update on this? Anything you need me to do?

[andriisoldatenko]

Could you please add unit tests?

[pcraciunoiu]

@andriisoldatenko do you have permissions to merge? I can look into tests.

I'm concerned because many people have forked this project, probably thinking it looks abandoned.