Update PHPUnit

[Deuchnord]

This PR updates the required version of PHPUnit and fixes an issue due to PHPUnit 7 (missing required attribute). Note: this PR drops the compatibility with PHP 7.0, so it is normal that the Travis CI fails on PHP 7.0.

[codecov[bot]]

Codecov Report

Merging #22 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #22   +/-   ##
=========================================
  Coverage     80.67%   80.67%           
  Complexity       43       43           
=========================================
  Files             1        1           
  Lines           119      119           
=========================================
  Hits             96       96           
  Misses           23       23

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update c8c4f19...1578f45. Read the comment docs.

[sebastianbergmann]

Thank you for your contribution. I appreciate the time you invested in preparing this pull request. However, I have decided not to merge it.