0.2.0 second pre-release

Thanks to advice from @dannyvankooten today, I was able to improve this library in a bunch of ways that I hadn't thought about myself. This release contains:

• Migration id methods are now static so the migrations can be queued without making an instance of them first, thus saving memory and increasing performance significantly.
• Core migrations are gone, as the DatabaseLogger is now responsible for its own table creation.
• Each implementation can define their own table name for the DatabaseLogger, so plugin vendors can specify their own table and log the state of their migrations in an isolated way.
• Rewritten the test suite, so it actually passes and works on Travis CI from PHP 5.6 up until 7.3.