Add gas check report in CI

[cgewecke]

Fixes #544, fixes #370.

Hi, opening for further discussion...

Adds a "coveralls for gas" CI hook which reports the changes in gas consumption between a PR and it's target branch. (It's agnostic about whether gas increases are good/bad and always succeeds.)

• Updates eth-gas-reporter to ^0.2.9
• Adds @codechecks/client dev dep (the CI service provider for the hook)
• Adds an additional CI job as a fast-finish / optional failure. (Because gas monitoring causes tests to run > 2X more slowly and extra info is not critical)

What this looks like:

• aragonOS new report on GitHub
• example report with changes on GitHub
• example Github CI UI
  

Further installation steps

• enable this project at codechecks.io (free for open source)
• paste a CC_SECRET env variable into the Travis settings for this repo

Things to consider...
The test suite here is industrial sized and this PR increases your CI overhead a bit. The gas reporter is a little slow because it makes extra data collection calls to the client. Hoping to change this soon and get it running as fast as normal tests. Even if you think this report would be useful you might prefer to wait for performance improvements, idk.

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Coverage remained the same at 99.125% when pulling 1e30755 on cgewecke:gas-check-ci into 4feb97a on aragon:next.

[sohkai]

@cgewecke This is awesome! Thank you so much for setting this up, the results look incredible!

The deployments section is super interesting, it should resolve our need for something like #370 as well :).

The test suite here is industrial sized and this PR increases your CI overhead a bit.

Haha I wouldn't call this test suite "industrial" by any means 😉 (this one is). It seems to run in ~15min right now, and I think that's very reasonable as an additional check.