Missing @throws BackoffException phpDoc

[webarchitect609]

There's no @throws BackoffException for several methods. So Php Storm thinks I'm coding wrong. "Exception BackoffException is never thrown" he told me today.

So I couldn't stand it and make PR #12 with this and a few more fixes. It would be nice if you can review them. The only file I left unchanged is changelog.

P.S.: Regarding failed test @ php 7.3. There's probably some kind of issue with Travis. I have php 7.3 installed locally and tested there. Everything's fine.

P.P.S: Thanks a lot for this lib! It saved me a lot of time at my work! :)