title: 31.0.0 is not a valid SDK Version. Option are 26.0.0 Unversioned
body: <p>I can no longer run my app after updating expo version to 31.0.0. I needed this upgrade to do due iOS version 12 support. Thou after trying to build the Android app it is no longer working.</p>
  
  <p>my package.json looks as below:</p>
  
  <pre><code>{
    "name": "myapp",
    "version": "0.1.0",
    "private": true,
    "devDependencies": {
      "react-native-scripts": "1.13.1",
      "jest-expo": "^31.0.0",
      "react-test-renderer": "16.3.0-alpha.1"
    },
    "main": "./node_modules/react-native-scripts/build/bin/crna-entry.js",
    "scripts": {
      "start": "react-native-scripts start",
      "eject": "react-native-scripts eject",
      "android": "react-native-scripts android",
      "ios": "react-native-scripts ios",
      "test": "jest"
    },
    "jest": {
      "preset": "jest-expo"
    },
    "dependencies": {
      "axios": "^0.18.0",
      "expo": "^31.0.4",
      "react": "16.5.0",
      "react-native": "https://github.com/expo/react-native/archive/sdk-31.0.1.tar.gz",
      "react-native-image-slider": "^2.0.3",
      "react-native-onesignal": "^3.1.4",
      "react-native-progress": "^3.4.0",
      "react-native-render-html": "^3.9.3",
      "react-navigation": "^1.5.11"
    }
  }
  </code></pre>
  
  <p>app.json looks like:</p>
  
  <pre><code>{
    "expo": {
      "sdkVersion": "31.0.0",
      "name": "myapp",
      "slug": "myapp",
      "version": "0.1.1",
      "isDetached": true,
      "detach": {
        "scheme": "expe6b74e69f7904f4b8961fc359e0dc1ce",
        "androidExpoViewUrl": "https://s3.amazonaws.com/exp-exponent-view-code/android-v2.4.0-sdk26.0.0-e63d9209-070c-4118-b06b-c60e82da0b66.tar.gz"
      },
      "android": {
        "package": "com.myapp"
      }
    }
  }
  </code></pre>
  
  <p>Android build.gradle app looks like this.:</p>
  
  <pre><code>android {
    compileSdkVersion 27
    buildToolsVersion '26.0.1'
  
    defaultConfig {
      applicationId 'com.ferrometal12'
      targetSdkVersion 27
      versionCode 1
      versionName '0.1.1'
      ndk {
        abiFilters 'armeabi-v7a', 'x86'
      }
      multiDexEnabled true
      testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
      // Deprecated. Used by net.openid:appauth
      manifestPlaceholders = [
        'appAuthRedirectScheme': 'host.exp.exponent'
      ]
    }
  </code></pre>
  
  <p>Any idea why this is not working on Android anymore? </p>
  
  <p>NOte: before i had expo 26.0.0 I also had targetSdkVersion to 26, after updating expo I also updated targetSdkVersion to 27</p>
  
  <p>UPDATE: This error occurs on emulator, I havent yet tried on real device, but I have the feeling that the same would be on real device too.</p>