Travis CI improperly configured; also, PR for callback insertion

[egitto]

I've submitted a PR that runs a callback function on every missing translation, which seems like a generically useful way for users to dynamically generate translation files (json, .po, whatever). At the moment it's held up by Travis, which is failing for reasons evidently not related to my PR:

https://travis-ci.org/webpack-contrib/i18n-webpack-plugin/jobs/325561169

ERROR: npm is known not to run on Node.js v4.3.2
Node.js 4 is supported but the specific version you're running has
a bug known to break npm. Please update to at least 4.7.0 to use this
version of npm. You can find the latest release of Node.js at https://nodejs.org/
The command "npm --version" failed and exited with 1 during .
Your build has been stopped.

Any suggestions concerning the callback or the Travis CI process would be appreciated; I'd rather not have to maintain a separate branch for this feature, especially since it's something other people would likely use. (see #60)