update php-code-coverage to 2.0

It seems that something was off with the version of php-code-coverage I had at the time of my previous contribution. One of the changes I made then are causing errors now with a clean checkout when text format is used.

It would be possible to fix that in the code, but it seems better to just update to a more recent version of php-code-coverage, where both text and html formats work as expected.