Packagist integration improvements

- adding `composer.lock` to repo (faster dependency resolution on Scrutinizer CI)
- adding branch alias to `composer.json` to ease version specification
- removing `minimum-stability` from `composer.json`
Adding more tests to increase coverage

- solving problems with too long lines
- code usage in test constructor wasn't reflected in coverage report
- expected exception wasn't reset after each test
- simplify exception catching in existing tests