Buildbot 0.9 support

[str4d]

Buildbot 0.9 replaced their ad-hoc API with a JSON-RPC REST interface. This PR adds support for transparently using the v2 API if available, and falling back to the v1 API.

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This change is 

[str4d]

I implemented and tested this as part of upgrading our CI system at https://ci.z.cash/ to Buildbot 0.9. However, I expect it needs significantly-more testing, especially to ensure that the v1 API code is unchanged.

[Manishearth]

r? @edunham or @aneeshusa

Probably don't have time myself to look through all this. May get to it eventually.

[str4d]

Oh, side-note: v2 support is currently broken without the fix in buildbot/buildbot#3180 (which I'm currently applying as a patch to buildbot 0.9.7 in our CI).

[edunham]

@str4d please make sure the refactor passes flake8 -- https://travis-ci.org/servo/homu/jobs/230962270

[cgwalters]

My vote would be removing the explicit BuildBot support - particularly now that (I believe) Rust no longer uses it. Instead, change your BuildBot configuration to write github status hooks. Then, no special support for BuildBot is necessary in Homu - all build systems are equal.

[bors-servo]

☔ The latest upstream changes (presumably #117) made this pull request unmergeable. Please resolve the merge conflicts.