Upgrade node and yarn versions

[josephpage]

Node 8.10.0 is the latest LTS, since node 6.x is near end of its "active" life.
(https://github.com/nodejs/Release)

Yarn 1.5.1 is the latest stable release.
(https://github.com/yarnpkg/yarn/releases)

@hone @schneems @jkutner your reviews are eagerly awaited !

Thanks !

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Edited since creation :

• node from 8.9.4 to 8.10.0 (2018-03-15)

[kfitzpatrick]

Looks like the HEROKU_API_USER is not set and CI is failing.

Thanks for this patch, I'm really excited to see this updated.

$ bundle exec rake hatchet:setup_travis
== Setting Up Travis ==
rake aborted!
KeyError: key not found: "HEROKU_API_USER"
/home/travis/build/heroku/heroku-buildpack-ruby/vendor/bundle/ruby/2.4.0/gems/heroku_hatchet-3.0.4/lib/hatchet/tasks.rb:9:in `fetch'

[jkutner]

@josephpage @kfitzpatrick don't worry about the CI failures. It requires a private token that is set on forked PRs (so as to not leak it). We can run the tests locally.

[schneems]

Thanks for the PR! I pushed this to another pr #721 to run tests. Once that is passing I can merge in, after that i'll be able to deploy sometime early next week likely.

[schneems]

Closed by #721 thanks for the PR!