Bump swiftmailer/swiftmailer from 5.4.8 to 6.0.2

[dependabot-preview]

Bumps swiftmailer/swiftmailer from 5.4.8 to 6.0.2.

Changelog

Sourced from swiftmailer/swiftmailer's changelog.

6.0.2 (2017-09-30)

• fixed DecoratorPlugin
• removed usage of getmypid()

6.0.1 (2017-05-20)

• fixed BC break that can be avoided easily

6.0.0 (2017-05-19)

• added Swift_Transport::ping()
• removed Swift_Mime_HeaderFactory, Swift_Mime_HeaderSet, Swift_Mime_Message, Swift_Mime_MimeEntity,
  and Swift_Mime_ParameterizedHeader interfaces
• removed Swift_MailTransport and Swift_Transport_MailTransport
• removed Swift_Encoding
• removed the Swift_Transport_MailInvoker interface and Swift_Transport_SimpleMailInvoker class
• removed the Swift_SignedMessage class
• removed newInstance() methods everywhere
• methods operating on Date header now use DateTimeImmutable object instead of Unix timestamp;
  Swift_Mime_Headers_DateHeader::getTimestamp()/setTimestamp() renamed to getDateTime()/setDateTime()
• bumped minimum version to PHP 7.0
• removed Swift_Validate and replaced by egulias/email-validator

5.4.9 (2018-01-23)

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

Commits

• 4123333 prepared the 6.0.2 release
• 34824a9 bug #1007 A possible fix for the issues #957 and #967 (zerkms)
• 1bc6d76 A possible fix for the issues #957 and #967
• 1a2f9df Merge branch '5.x'
• 93df08e minor #995 [Docs] Converted mysql_* into PDO calls (Oire)
• 88cd352 Docs: Converted mysql_* into PDO calls. See #994
• bd3da52 Merge branch '5.x'
• 0dcea92 minor #990 Updated Homepage and Documentation Link (jgacuca567)
• 64bbfcb Updated Homepage and Documentation Link
• f61bd08 fixed URL in composer.json
• Additional commits viewable in compare view

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