Use newer versions of Node and NPM
Use newer versions of Node and NPM

Travis CI installs Node v0.10.36 with NPM v1.4.28 by default, which is really old and doesn't support using authentication tokens from ~/.npmrc, so we need to install something newer for npm publishing to work. Using Node 6.x as it is the latest LTS release.

jonheslop/govuk_template,jonheslop/govuk_template,alphagov/govuk_template,alphagov/govuk_template,alphagov/govuk_template,alphagov/govuk_template,jonheslop/govuk_template,jonheslop/govuk_template,jonheslop/govuk_template,alphagov/govuk_template