Add Node 8 and Node 9 to continuous integration testing

[cooltoast]

Hi @guyonroche,

I appreciate your work on this package! I use it regularly in different node environments, and I think using simple continuous integration would be very helpful for me (and possibly others) to understand this package's node compatibility, and for ensuring tests pass for pull requests and new features.

In a previous PR I've added a .travis.yml file that TravisCI can use. It's free of charge, and the good news is that the tests pass in Node versions 6-9!

Please consider activating this repo in TravisCI: https://docs.travis-ci.com/user/getting-started/
Upon the next pushed commit, it'll immediately build and run the tests you've written.

Thanks!

[cooltoast]

Hi @guyonroche

Is there an issue with this PR? I see you've been merging PR's recently and just wanted to check in

[guyonroche]

Hi @cooltoast - I'm reviewing this PR right now - as mentioned, I am currently overworked with very little time to spare. I'm trying to set aside a little time each day to catch up with all the PRs... they kinda sneaked up on me!
Glad you like this lib :-)

[cooltoast]

Hey @guyonroche thanks for merging!