chore: bump travis nodejs version to 8

[jelbourn]

@crisbeto I vaguely recall there was some issue w/ Angular preventing use of node 8, but I'm guessing it's likely resolved now.

[crisbeto]

The issue was with a regression in Node 8.8 that ended up breaking Travis (see nodejs/node#16484). It looks like the fix landed in 8.8.1. We should probably be more explicit about the version that we're setting since Travis might not have all of them.

[jelbourn]

Travis docs say that it should always install the latest version when you specify a major number:
https://docs.travis-ci.com/user/languages/javascript-with-nodejs#Specifying-Node.js-versions

We can see how this goes before getting more specific

[crisbeto]

LGTM

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