title: expo init is throwing error regarding missing export main in is-promise
body: <p>I have installed expo globally using npm on windows 10.
  while i am running the basic script - expo init app-name, i am receiving the following error :</p>
  
  <pre><code>Error: No valid exports main found for 'C:\Users\Vaibhav Singh\AppData\Roaming\npm\node_modules\expo-cli\node_modules\is-promise'
      at resolveExportsTarget (internal/modules/cjs/loader.js:618:9)
      at applyExports (internal/modules/cjs/loader.js:499:14)
      at resolveExports (internal/modules/cjs/loader.js:548:12)
      at Function.Module._findPath (internal/modules/cjs/loader.js:650:22)
      at Function.Module._resolveFilename (internal/modules/cjs/loader.js:948:27)
      at Function.Module._load (internal/modules/cjs/loader.js:854:27)
      at Module.require (internal/modules/cjs/loader.js:1023:19)
      at require (internal/modules/cjs/helpers.js:72:18)
      at Object.&lt;anonymous&gt; (C:\Users\Vaibhav Singh\AppData\Roaming\npm\node_modules\expo-cli\node_modules\run-async\index.js:3:17)
      at Module._compile (internal/modules/cjs/loader.js:1128:30) 
  </code></pre>
  
  <p>This is the is-promise file which the error refers :</p>
  
  <pre><code>module.exports = isPromise;
  module.exports.default = isPromise;
  
  function isPromise(obj) {
    return !!obj &amp;&amp; (typeof obj === 'object' || typeof obj === 'function') &amp;&amp; typeof obj.then === 'function';
  }
  </code></pre>
  
  <p>I haven't modified it. 
  I </p>