Bump swiftmailer/swiftmailer from 5.4.6 to 5.4.12

[dependabot-preview]

Bumps swiftmailer/swiftmailer from 5.4.6 to 5.4.12.

Changelog

Sourced from swiftmailer/swiftmailer's changelog.

5.4.12 (2018-07-31)

• fixed typo

5.4.11 (2018-07-31)

• fixed startTLS support for PHP 5.6-

5.4.10 (2018-07-27)

• fixed startTLS only allowed tls1.0, now allowed: tls1.0, tls1.1, tls1.2

5.4.9 (2018-01-23)

• no changes, last version of the 5.x series

5.4.8 (2017-05-01)

• fixed encoding inheritance in addPart()
• fixed sorting MIME children when their types are equal

5.4.7 (2017-04-20)

• fixed NTLMAuthenticator clobbering bcmath scale

Commits

• 181b89f prepared the 5.4.12 release
• dccdd7c updated CHANGES
• d96063e fixed typo
• 7c6640e prepared the 5.4.11 release
• 5a82b2e updated CHANGES
• d9aed42 refactored code
• 421299e bug #1112 wxu: Fix release version is not compatible with low version PHP (wxu)
• c14b7af wxu: Fix release version is not compatible with low version PHP
• dd71cc1 prepared the 5.4.10 release
• 5fbe82f Allow explicit tls1.0, tls1.1, tls1.2 for startTLS
• Additional commits viewable in compare view

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