add php 7.3 to travis

[andreybolonin]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Fixed tickets	fixes #issuenum
Related issues/PRs	#issuenum
License	MIT

What's in this PR?

Explain the contents of the PR.

Why?

Which problem does the PR fix?

Example Usage

$foo = new Foo();

// Now we can do
$foo->doSomething();

// Remove this section if not needed
~~~

#### BC Breaks/Deprecations

Describe BC breaks/deprecations here (Remove this section if not needed).

#### To Do

- [ ] Create tests

[jens1o]

Awesome, thanks!