Bump minimum rust version

[jrraymond]

Are there features fixedbitset would benefit from but cannot use without bumping the minimum rust version?

[bluss]

I've added a bug label breaking-change which is a good way to track such changes, and also to save and catch up on them when it's a good time for breaking changes.

[bluss]

There are no dependencies or dev-deps even, so we're quite free. Using quickcheck could be good, and then we would be in the "rat race", it requires Rust 1.22. at the moment. I don't have a strong urge for any major jumps.

• Using RangeBounds trait from std would be neat, but that's all the way up at Rust 1.28.

[bluss]

@jrraymond I'd propose transferring this crate into the petgraph organization. What do you think? Do you want to join the org, too? (It's entirely optional)

[jrraymond]

@bluss I think that's a good idea. I'd like to join the org too.

[bluss]

Fixed by #35 - now Rust 1.31.