remove Node.js v7 in travis-ci
remove Node.js v7 in travis-ci

This project uses `contributor-faces`, it includes `gh-got`.

And `gh-got` require Node v8 so `travis-ci` will have the error.

So I have 2 option:

- Remove Nodejs v7 in `.travis.yml`
- Or install `contributor-faces` in your machine(require Node >= 8) and sometime run `contributor-faces .` in your project to update list contributor.
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