Drop PHP 7.1 support; PHP 7.2 is now required.

Drop PHP 7.0 support; PHP 7.1 is now required.

Add GlobalsMetricFetcher class.

New requirements:

• PHP 7.0+
• PHPUnit 6.0+

• print out detected problems after finishing the whole test suite (previously it was getting printed straight-away)

• support better formatting of non-numeric metrics
• use PHPUnit 5.6 for testing

• support disabling the naughty test detector by setting env variable DISABLE_NAUGHTY_TEST_DETECTOR to 1 or true

• support evaluation of test-level, test-suite-level and/or global level

• Move satooshi/php-coveralls to dev dependencies.
• Improve README.md.