Drop PHP 7.0 support, add empty defaults for bool/int/float/string return types

Merge pull request #6 from ifwe/update-dependencies

 Require php 7.1 or newer. Add defaults for int/float/string/bool

Create a release for PHP 7.0 patches

Merge pull request #4 from ifwe/update-phockito

Throw a TypeError for mocks with invalid return types, upgrade phpunit dependency