Install maildev from GitHub

[groenroos]

maildev has been resurrected, and the outdated packages have been updated. However, they seem to have some sort of issue with publishing the new version on npm (maildev/maildev#390).

Until such time that this is updated on npm, we can get the new version direct from GitHub, to address the vulnerabilities introduced by their outdated dependencies.

Closes #420

[codecov]

Codecov Report

Merging #442 (35ee38e) into master (1a55427) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #442   +/-   ##
=======================================
  Coverage   98.07%   98.07%           
=======================================
  Files          37       37           
  Lines        4668     4668           
=======================================
  Hits         4578     4578           
  Misses         90       90           

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