phpunit: Speed up code coverage calculation

[JDGrimes]

We used to run this on HHVM, which would complete in about 12 minutes. Because we dropped HHVM (see #219), we switched it to run on PHP 7.0 instead. However, it takes more than 40 minutes, causing the build to error since the maximum run time is exceeded.

Perhaps we could run this on 7.1, and that would be faster, or there is some configuration option that can speed this up. Otherwise I think that we might want to bring HHVM back just to run the code coverage checks.

[JDGrimes]

I found sebastianbergmann/phpunit#2489, but I don't think that it is related, because we are using PHPUnit 5.7.20, and that bug was in 5.7.10, and fixed in 5.7.14.

The version of PHPUnit that was running on HHVM was 4.5.0.

[JDGrimes]

Running on PHP 7.1 didn't make a difference: https://travis-ci.org/WordPoints/wordpoints/jobs/238799864