Add gas check report in CI

[cgewecke]

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. (The hook is agnostic about whether gas increases are good/bad - it 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 CI hook would be useful you might prefer to wait for performance improvements, idk.