Allow PHPUnit 7

[o5]

#11 (comment)

[ondrejmirtes]

Hi, this PR had number of missing things so I decided to come up with my own approach: 852411f

Specifically:

• The change had to be based on 0.9.3 and not on master so it could be released quickly
• I write the lower version first and upper second: ^6.3 || ^7.0
• The Travis build had to be updated so all version constraints were tested.

It was released as 0.9.4 just a few moments ago.

[o5]

@ondrejmirtes I can confirm, version 0.9.4 resolves my problem. Thank you!