Tooling improvements

[tfrommen]

Hey! 👋

This PR includes a number of unrelated improvements to various tooling:

• PHP_CodeSniffer: Reference HM standard by name, not path.
• PHPUnit: Remove default attribute (7.5 config - 9.2 config), sort attributes, and improve coverage setup.
• Simplify and improve browsers list (see best-practices).
• Composer: Simplify running Composer scripts.
• Composer: Sort packages.
• Remove empty dependencies section.
• Travis: Ensure installing PHP dependencies is quick and doesn't fail the build.
• Use Composer "test" script as canonical way to run PHP tests.
• Test bootstrap: Extract and align base directory.
• PHPUnit: Replace deprecated "assertInternalType" with "assertIsInt" (see Deprecate assertInternalType() and assertNotInternalType() sebastianbergmann/phpunit#3369).

In case you don't like a partciular thing, feel free to chery-pick. 🍒

[kadamwhite]

Thanks for this @tfrommen !