Not compiling

Attempted compilation with:
```
cargo +nightly run
```
Removed features `["legion-ecs", "nightly"]` from `amethyst` dependency in:
```
Cargo.toml
core/Cargo.toml
assets/Cargo.toml
```
because they are no longer features of `amethyst`.

Used the latest `master` of both `amethyst` and `amethyst_imgui`. Also tried with crate versions `0.15.0` and `0.7.0` respectively.