Show test coverage directly after running test

After a chat on IRC with @cboden and @clue, they seemt positive, I've made this change to the existing PR. (And the other --coverage-text PR's I've made yesterday.) Pulling it in line with @psychoticmeow's PR's like https://github.com/reactphp/socket-client/pull/8.

In my opinion `composer install` should install all PHP packages required to develop a package, including phpunit. Not everyone may have it installed, but more likely installs can be outdated and pulling it in likes this ensures everyone has the correct version and can run the tests.