Fix failing test

[sindresorhus]

https://travis-ci.org/yeoman/update-notifier/jobs/512463652

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[LitoMore]

I found the bug is here:

https://github.com/yeoman/update-notifier/blob/master/index.js#L120

The isNpm().isNpm returned a undefined.

Looks the ava is using the is-npm@1.0.0 for test.

[LitoMore]

We could mark as failing test temporary. Then wait for ava update the update-notifier to next version.

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