feat(deps): add support for node@8

[kellyjensen]

Add support for nodejs v8.

Updated package.json, .travis.yml, appveyor.yml and installation docs.

Fixes #2754

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again.

[kellyjensen]

Thanks @pedrotcaraujo for providing the blueprint for upgrading the supported node version in your PR #2571.

[kellyjensen]

Try google CLA again.

[shinnn]

Try google CLA again.

This doesn't help. The most important point of the @googlebot's comment is:

If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)?

[kellyjensen]

Would it be better to close this PR and submit a new one ensuring that my email is configured correctly?

[mgol]

@kellyjensen it should be enough for you to amend the commit with the correct name & mail:

git commit --amend --author="Your Name <your.email@example.com>"

and then git push --force.

[kellyjensen]

Thank you! I will try that.

[googlebot]

CLAs look good, thanks!

[shinnn]

Thanks to #2767, rebasing from the latest master will probably fix all failing tests.

[kellyjensen]

Yay CI passed!

[qswitcher]

+1 this is blocking me from configuring npm with engine-strict enabled.

[kellyjensen]

Is there anything else I need to do? Just checking.

[shaiksphere]

Is there an update on this one? This is blocking the usage of karma on the Node 8 based projects of ours.

[dignifiedquire]

Thank you