update-notifier using old boxen which hangs on some machines

[dankegel]

When run in background (or, more precisely, with stderr redirected to a unix domain socket),
'npm install' hangs for me on mac os x 10.11. This is relatively recent behavior; it affects
npm 5.3.0, but not npm 3.10.8. I can work around it with 'npm install -g npm@3.10.8'.

The culprit appears to be the older version of boxen required by package.json, i.e. 1.0.0:
https://github.com/yeoman/update-notifier/blob/master/package.json#L36

More recent boxen require more recent versions of term-size, which avoid the hang.

See also npm/npm#18759

[SBoudrias]

Mind sending a PR?

[sindresorhus]

We target ^1.0.0 which matches the latest version of Boxen, 1.2.1, so bumping the version is pretty moot. But I'll bump the version since I'm gonna do a new version today anyway.