minor #1018 add php 7.2 (Rotzbua)
minor #1018 add php 7.2 (Rotzbua)

This PR was merged into the 6.0-dev branch.

Discussion
----------

add php 7.2

<!-- Please fill in this template according to the PR you're about to submit. -->

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | not really
| Doc update?   | no
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | #... <!-- #-prefixed issue number(s), if any -->
| License       | MIT

add php 7.2 to travis build

Commits
-------

583d36f add php 7.2

jamesggordon/swiftmailer,swiftmailer/swiftmailer,voku/swiftmailer,Rotzbua/swiftmailer,gencer/swiftmailer,voku/swiftmailer,jamesggordon/swiftmailer,gencer/swiftmailer,c960657/swiftmailer,c960657/swiftmailer,michabbb-backup/swiftmailer,Rotzbua/swiftmailer,swiftmailer/swiftmailer,michabbb-backup/swiftmailer