Update txthinking/mailer to its master branch

The last tagged release (v2.0.1) is from 2018 and lacks important fixes.
Track dev-master (pinned to an exact commit via composer.lock) to pick up
the m

Update txthinking/mailer to its master branch

The last tagged release (v2.0.1) is from 2018 and lacks important fixes.
Track dev-master (pinned to an exact commit via composer.lock) to pick up
the modernized TLS handling and the new ability to skip certificate
verification.

Ignore the .github directory that the dev-master archive ships with.

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