Update to amethyst 0.15

[mbStavola]

Amethyst recently published a 0.15 release, which unfortunately breaks this crate in a few ways:

Thankfully, it's only because of a transitive crate mismatch which we can fix by updating from Amethyst 0.14 to 0.15. No other changes are necessary.

Since this is likely a breaking change, I thought it might be appropriate to bump this crate to 0.7.0 instead of 0.6.2.

[Deedasmi]

+1, works fine. EDIT: I'd actually probably suggest that the version number match the Amethyst version, in this case 0.15.

[FireFlyForLife]

I would also be in favor of bumping to 0.15.

[mbStavola]

@Deedasmi @FireFlyForLife While I agree in principle, it doesn't appear to have been the standard for this crate.

If a maintainer is in favor, I'll gladly bump the version.

[jaynus]

closing in favor of #14