Fix installation step in JS client readme

In the JS client readme we currently say:

After installing Node.js, use npm to install the nuxeo package:

$ npm install nuxeo --save

or, for the latest 1.x.x version:

$ npm install nuxeo@1 --save

But using $ npm install nuxeo --save actually installs the last version published on npm, and it currently is the 1.x.

The documentation should be updated to always precise the version to be installed.