make FloatParameter accessible just as the others

Missed this line, when I [added](https://github.com/spotify/luigi/commit/3afa87f5f19c4a419f3180cb2fe6c34f3b11f404) `FloatParameter`. (And yes, I read the comment above the block: [TODO: how can we get rid of these?](https://github.com/miku/luigi/blob/8f56863c80258dda59e2b9422dadef2cd3113c42/luigi/__init__.py#L39) - but I have no answer ATM.)