add php 7.3 to travis

[andreybolonin]

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I'm working on first LTS release of Deployer.
Please, consider supporting Deployer development on Patreon page:

https://www.patreon.com/deployer

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Q	A
Bug fix?	Yes or No
New feature?	Yes or No
BC breaks?	Yes or No
Deprecations?	Yes or No
Fixed tickets	N/A or xx

Do not forget to add notes about your changes to CHANGELOG.md

Easiest way to do it, by running next command:

php bin/changelog