Travis removes encrypted env variables for external PR

[dlespiau]

The build currently needs NPM_TOKEN defined and that variable is defined in Travis.

I think it's actually quite a pain to have this .npmrc file with an undefined token variable for something that is only used for publishing releases on npm. A thing we could do is to only generate that fine before deploying:

 echo "//registry.npmjs.org/:_authToken=$NPM_TOKEN" > .npmrc