minimum php version is 5.6 now

[dbu]

• use short array notation
• use ::class pseudo-constant
• use createMock for phpunit
• simplify tests setup

[dbu]

@wouterj i now try to use the phpunit-bridge as well. but it looks like phpunit is not installed correctly - can you spot what i do wrong?

[dbu]

ready to review and merge \o/