Upgrade version of gui-diff to 0.6.7

[amithgeorge]

gui-diff 0.6.6 depends on ordered instead of flatland/ordered. This causes various conflicts with other libraries that depend on flatland/ordered. The library author recommends using the latter. gui-diff 0.6.7 supports this.

Raised a PR that fixes this - #11

[raycheung]

I share the same problem, thank you for the PR!