title: Cannot upgrade npm to the latest version on Mac
body: <p>No matter what I've tried, I can't update npm to the latest version (currently <code>3.10.7</code>) on my Mac. The install seems to work, in terms of pulling down the latest version, but the latest version is stuck at 3.8.0. Here are all the details I can pull up.  </p>
  
  <p>system: <code>Mac OS X El Capitan, Version 10.11.6</code>  </p>
  
  <p>command: <code>node -v</code><br>
  results: <code>v6.6.0</code>  </p>
  
  <p>command: <code>npm -v</code><br>
  results: <code>3.8.0</code>  </p>
  
  <p>command: <code>npm cache clean</code><br>
  results: <em>nothing</em></p>
  
  <p>command: <code>which npm</code><br>
  results: <code>Users/wrburgess/.npm-packages/bin/npm</code>  </p>
  
  <p>command: <code>npm install -g npm@latest</code><br>
  results:  </p>
  
  <p><code>
  /usr/local/bin/npm -&gt; /usr/local/lib/node_modules/npm/bin/npm-cli.js
  /usr/local/lib
  ├── abbrev@1.0.9
  ├── aproba@1.0.4
  ├── fstream@1.0.10
  ├─┬ fstream-npm@1.1.1
  │ └─┬ fstream-ignore@1.0.5
  │   └── minimatch@3.0.3
  ├─┬ glob@7.0.5
  │ └── minimatch@3.0.3
  ├── graceful-fs@4.1.5
  ├── has-unicode@2.0.1
  ├── lodash.clonedeep@4.4.1
  ├── lodash.union@4.5.0
  ├── lodash.uniq@4.4.0
  ├── lodash.without@4.3.0
  ├── minimatch@3.0.3
  ├─┬ node-gyp@3.4.0
  │ └── minimatch@3.0.3
  ├─┬ npm@3.10.7
  │ └─┬ init-package-json@1.9.4
  │   └─┬ glob@6.0.4
  │     └── minimatch@3.0.3
  ├── npm-package-arg@4.2.0
  ├── npm-user-validate@0.1.5
  ├── read-package-tree@5.1.5
  ├── readable-stream@2.1.4
  ├── request@2.74.0
  ├── rimraf@2.5.4
  ├── which@1.2.10
  └── wrappy@1.0.2
  </code></p>
  
  <p>command: <code>npm cache clean</code><br>
  results: <em>nothing</em></p>
  
  <p>command: <code>npm -v</code><br>
  results: <code>3.8.0</code></p>
  
  <p>Any ideas?</p>