Use f64 instead of float.

`float` has been removed from the language. SQLite documents their REAL type
as being an [f64](), so use that. Exposes a correctness bug where Rust
wouldn't pick f32 as the float type.

[f64] - http://sqlite.org/datatype3.html
Merge pull request #53 from cmr/float-removal

Use f64 instead of float.