Make Node.js 6 the minimum version.

Also, be consistent with Node.js' name.

.travis.yml

@@ -4,7 +4,7 @@ language: node_js
 git:
   depth: 3
 node_js:
-  - "4"
+  - "6"
   - "8"
 before_install:
   - if [[ `npm -v` != 5* ]]; then npm install -g npm@5; fi

docs/4.0/getting-started/build-tools.md

@@ -12,7 +12,7 @@ Bootstrap uses [NPM scripts](https://docs.npmjs.com/misc/scripts) for its build
 
 To use our build system and run our documentation locally, you'll need a copy of Bootstrap's source files and Node. Follow these steps and you should be ready to rock:
 
-1. [Download and install Node](https://nodejs.org/download/), which we use to manage our dependencies.
+1. [Download and install Node.js](https://nodejs.org/download/), which we use to manage our dependencies.
 2. Navigate to the root `/bootstrap` directory and run `npm install` to install our local dependencies listed in [package.json]({{ site.repo }}/blob/v{{ site.current_version }}/package.json).
 3. [Install Ruby][install-ruby], install [Bundler][gembundler] with `gem install bundler`, and finally run `bundle install`. This will install all Ruby dependencies, such as Jekyll and plugins.
   - **Windows users:** Read [this guide](https://jekyllrb.com/docs/windows/) to get Jekyll up and running without problems.

docs/4.0/getting-started/download.md

@@ -50,7 +50,7 @@ Pull in Bootstrap's **source files** into nearly any project with some of the mo
 
 ### npm
 
-Install Bootstrap in your Node powered apps with [the npm package](https://www.npmjs.com/package/bootstrap):
+Install Bootstrap in your Node.js powered apps with [the npm package](https://www.npmjs.com/package/bootstrap):
 
 {% highlight sh %}
 npm install bootstrap@{{ site.current_version }}

docs/4.0/getting-started/webpack.md

@@ -8,7 +8,7 @@ toc: true
 
 ## Installing Bootstrap
 
-[Install bootstrap]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/download/#npm) as a node module using npm.
+[Install bootstrap]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/download/#npm) as a Node.js module using npm.
 
 ## Importing JavaScript
 

package.json

@@ -109,7 +109,7 @@
     "workbox-build": "^2.0.1"
   },
   "engines": {
-    "node": ">=4"
+    "node": ">=6"
   },
   "files": [
     ".babelrc",