Bump to v0.4.0

Next version number is needed since we removed higher order derivs and
made F completely generic.

Bump version to 0.3.2

Bump to v0.3

Bump version to v0.2.2

Bump to v0.1.9

Bump to v0.1.8

Added feature gated cgmath::BaseFloat impls

This includes the 3 impls to make F implement BaseFloat from cgmath.

Small modernizations

Cleanup README with Num -> F rename.
Migrate to Rust 2018.

Bump version to 0.1.4

Bump version to 0.1.3