Modernize to PHP 7.1

[phpfui]

Changes include:

• Add scalars to method parameters where ever possible.
• Add return types where ever possible.
• Upgrade to PHPUnit 7 or higher and remove LegacyPHPUnit package
• Update readme.md to reflect PHP version support
• Update github actions to support only PHP 7.1 and higher.
• Added PHPStan to level 6 for src and tests directories.
• Ran PHP-CS-Fixer on src and tests directories.
• Fixed code for testing read method to test against bool, since read always returns string.
• Removed useless comments @param int $length, but kept @param int $length number of bytes to read.
• Used @inheritdoc where is made sense.

[zbateson]

Thanks again :)