title: Issue installing `flutter_twitter_login` and `TwitterKit` with Flutter
body: <p>I'm trying to build an app made in Flutter onto iOS right now.  However, I'm getting issues including all the dependencies (shown below).  When I build the app for the first time, it runs <code>pod install</code> and everything goes fine until it starts running pre install hooks, where I get a big block of error messages (below).  What I can tell from this is I have some sort of issue with <code>flutter_twitter_login</code> (one of the error messages says something about twitter), and that there's an issue with my podfile (the error message asks me to call out a minimum version of iOS).  Additionally, if I try to run it again (changing the podfile so it doesn't complain about the minimum version thing), it gives me an error saying it can't find <code>&lt;TwitterKit/TwitterKit.h&gt;</code> (the full error message below).</p>
  
  <p>I tried to replicate this by building the <a href="https://pub.dartlang.org/packages/flutter_twitter_login#-example-tab-" rel="noreferrer">example program</a> for <code>flutter_twitter_login</code>, and it was able to build find with the exact same <code>pubspec.yaml</code>.  I also noticed that in <code>/path/to/project/ios/Pods/TwitterKit/iOS/TwitterKit.framework/Headers/</code>, there is no <code>TwitterKit.h</code> just like the error says, but in the project that I made to replicate the error there is.</p>
  
  <p>So I think there's something bad going on with how cocoapods is installing TwitterKit, and the fact that I'm getting complaints about not calling out a minimum iOS version in the auto-generated podfile suggests that there's something funny with how that's being made.  However, I'm not really sure how to proceed with debugging this since the problem evidently isn't with <code>pubspec.yaml</code>, since the file that I have can successfully build other projects, and my understanding is that the podfile comes directly from <code>pubspec.yaml</code>.</p>
  
  <p><em>Code snippets and error messages</em></p>
  
  <p>I have this in <code>pubspec.yaml</code> (everything other than this is from the template):</p>
  
  <pre><code>dependencies:
    flutter:
      sdk: flutter
    firebase_core: ^0.2.5
    firebase_analytics: ^1.0.4
    http: "^0.11.3+17"
    firebase_auth: 0.6.2+1
    google_sign_in: ^3.2.1
    flutter_twitter_login: "^1.1.0"
    fluttertoast: ^2.0.7
    flutter_svg: ^0.6.1+1
  </code></pre>
  
  <p>Big block of error messages while cocoapods is running pre install hooks:</p>
  
  <pre><code>      - Running pre install hooks
      [!] The 'Pods-Runner' target has transitive dependencies that include static binaries: (/path/to/project/ios/Pods/TwitterKit/iOS/TwitterKit.framework)
  
      /Library/Ruby/Gems/2.3.0/gems/cocoapods-1.5.3/lib/cocoapods/installer/xcode/target_validator.rb:81:in `block (2 levels) in verify_no_static_framework_transitive_dependencies'
      /Library/Ruby/Gems/2.3.0/gems/cocoapods-1.5.3/lib/cocoapods/installer/xcode/target_validator.rb:73:in `each'
      /Library/Ruby/Gems/2.3.0/gems/cocoapods-1.5.3/lib/cocoapods/installer/xcode/target_validator.rb:73:in `block in verify_no_static_framework_transitive_dependencies'
      /Library/Ruby/Gems/2.3.0/gems/cocoapods-1.5.3/lib/cocoapods/installer/xcode/target_validator.rb:70:in `each'
      /Library/Ruby/Gems/2.3.0/gems/cocoapods-1.5.3/lib/cocoapods/installer/xcode/target_validator.rb:70:in `verify_no_static_framework_transitive_dependencies'
      /Library/Ruby/Gems/2.3.0/gems/cocoapods-1.5.3/lib/cocoapods/installer/xcode/target_validator.rb:36:in `validate!'
      /Library/Ruby/Gems/2.3.0/gems/cocoapods-1.5.3/lib/cocoapods/installer.rb:415:in `validate_targets'
      /Library/Ruby/Gems/2.3.0/gems/cocoapods-1.5.3/lib/cocoapods/installer.rb:118:in `install!'
      /Library/Ruby/Gems/2.3.0/gems/cocoapods-1.5.3/lib/cocoapods/command/install.rb:41:in `run'
      /Library/Ruby/Gems/2.3.0/gems/claide-1.0.2/lib/claide/command.rb:334:in `run'
      /Library/Ruby/Gems/2.3.0/gems/cocoapods-1.5.3/lib/cocoapods/command.rb:52:in `run'
      /Library/Ruby/Gems/2.3.0/gems/cocoapods-1.5.3/bin/pod:55:in `&lt;top (required)&gt;'
      /usr/local/Cellar/cocoapods/1.5.3/libexec/bin/pod:22:in `load'
      /usr/local/Cellar/cocoapods/1.5.3/libexec/bin/pod:22:in `&lt;main&gt;'
  
  Error output from CocoaPods:
  ↳
  
      [!] Automatically assigning platform `ios` with version `8.0` on target `Runner` because no platform was specified. Please specify a platform for this target in your Podfile. See `https://guides.cocoapods.org/syntax/podfile.html#platform`.
  </code></pre>
  
  <p>Can't find <code>TwitterKit.h</code>:</p>
  
  <pre><code>Error output from Xcode build:
  ↳
      ** BUILD FAILED **
  
  
  Xcode's output:
  ↳
      /path/to/flutter/.pub-cache/hosted/pub.dartlang.org/flutter_twitter_login-1.1.0/ios/Classes/TwitterLoginPlugin.m:2:9: fatal error: 'TwitterKit/TwitterKit.h' file not found
      #import &lt;TwitterKit/TwitterKit.h&gt;
              ^~~~~~~~~~~~~~~~~~~~~~~~~
      1 error generated.
  </code></pre>