Update swiftmailer/swiftmailer requirement to ~6.1.2

[dependabot-preview[bot]]

Updates the requirements on swiftmailer/swiftmailer to permit the latest version.

Changelog

*Sourced from [swiftmailer/swiftmailer's changelog](https://github.com/swiftmailer/swiftmailer/blob/master/CHANGES).* > 6.1.2 (2018-07-13) > ------------------ > > * handled recipient errors when pipelining > > 6.1.1 (2018-07-04) > ------------------ > > * removed hard dependency on an IDN encoder > > 6.1.0 (2018-07-02) > ------------------ > > * added address encoder exceptions during send > * added support for bubbling up authenticator error messages > * added support for non-ASCII email addresses > * introduced new dependencies: transport.smtphandlers and transport.authhandlers > * deprecated Swift_Signers_OpenDKIMSigner; use Swift_Signers_DKIMSigner instead > * added support for SMTP pipelining > * added Swift_Transport_Esmtp_EightBitMimeHandler > * fixed startTLS only allowed tls1.0, now allowed: tls1.0, tls1.1, tls1.2 > > 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) > ... (truncated)

Commits

- See full diff in [compare view](https://github.com/swiftmailer/swiftmailer/commits/v6.1.2)

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Superseded by #13.