title: When installing Expo, I get 'permission denied' errors
body: <p>When i try to install the Expo SDK, I get permission errors:</p>
  
  <pre><code>sudo npm install exp --global
  
  &gt; @expo/traveling-fastlane-darwin@1.2.5 preinstall /usr/local/lib/node_modules/exp/node_modules/@expo/traveling-fastlane-darwin
  &gt; node platform.js
  
  /usr/local/bin/exp -&gt; /usr/local/lib/node_modules/exp/bin/exp.js
  
  &gt; spawn-sync@1.0.15 postinstall /usr/local/lib/node_modules/exp/node_modules/spawn-sync
  &gt; node postinstall
  
  
  &gt; @expo/ngrok@2.3.0 postinstall /usr/local/lib/node_modules/exp/node_modules/@expo/ngrok
  &gt; node ./postinstall.js
  
  ngrok - error unpacking binary { Error: EACCES: permission denied, open '/usr/local/lib/node_modules/exp/node_modules/@expo/ngrok/bin/ngrok'
    errno: -13,
    code: 'EACCES',
    syscall: 'open',
    path: '/usr/local/lib/node_modules/exp/node_modules/@expo/ngrok/bin/ngrok' }
  npm WARN optional SKIPPING OPTIONAL DEPENDENCY: @expo/traveling-fastlane-linux@1.2.5 (node_modules/exp/node_modules/@expo/traveling-fastlane-linux):
  npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for @expo/traveling-fastlane-linux@1.2.5: wanted {"os":"win32,linux","arch":"any"} (current: {"os":"darwin","arch":"x64"})
  
  npm ERR! code ELIFECYCLE
  npm ERR! errno 1
  npm ERR! @expo/ngrok@2.3.0 postinstall: `node ./postinstall.js`
  npm ERR! Exit status 1
  npm ERR! 
  npm ERR! Failed at the @expo/ngrok@2.3.0 postinstall script.
  npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
  
  npm ERR! A complete log of this run can be found in:
  npm ERR!     /Users/chris/.npm/_logs/2018-01-27T10_59_33_236Z-debug.log
  </code></pre>
  
  <p>I'm using npm 5.5.1 + node v8.9.1 + macOS high sierra</p>