Publish version 0.8

[mitchmindtree]

Includes:

• Update petgraph to 0.6 Update to petgraph 0.6 #33
• Fix for issue where loops weren't detected as cycles Daggy not detecting loop cycles #28

Closes #34

@azriel91 thanks for the ping!

For future reference, if anyone's after a new version, feel free to open
a PR with a version bump - I've think I've setup this repo to
auto-publish :)