Add php 7.3 to travis builds

[Xerkus]

Provide a narrative description of what you are trying to accomplish:

• Are you fixing a bug?

  • Detail how the bug is invoked currently.
  • Detail the original, incorrect behavior.
  • Detail the new, expected behavior.
  • Base your feature on the master branch, and submit against that branch.
  • Add a regression test that demonstrates the bug, and proves the fix.
  • Add a CHANGELOG.md entry for the fix.

• Are you creating a new feature?

  • Why is the new feature needed? What purpose does it serve?
  • How will users use the new feature?
  • Base your feature on the develop branch, and submit against that branch.
  • Add only one feature per pull request; split multiple features over multiple pull requests
  • Add tests for the new feature.
  • Add documentation for the new feature.
  • Add a CHANGELOG.md entry for the new feature.

• Is this related to quality assurance?

• Is this related to documentation?