title: Can not install FBSDKCoreKit old version after run `pod repo update`
body: <p>I have a swift 3 project. I try to upgrade to swift 4.2 and it's required me to run <code>pod repo update</code> when I run <code>pod install</code>. I did it and <code>pod install</code> worked</p>
  
  <p>But the are some trouble then I have back to swift 3.
  Now I run <code>pod install</code> it's install new version of FBSDKCoreKit</p>
  
  <pre><code>Installing FBSDKCoreKit (5.4.1)
  Installing FBSDKLoginKit (5.4.1)
  Installing FBSDKShareKit (5.4.1)
  </code></pre>
  
  <p>That is not compatible with my swift 3 project.
  I want to <code>FBSDKCoreKit (4.40.0)</code> or older.</p>
  
  <p>I have changed Podfile to </p>
  
  <pre><code>target 'EPARK' do
    # Comment the next line if you're not using Swift and don't want to use dynamic frameworks
    use_frameworks!
  
    # Pods for EPARK
  pod 'FacebookCore', '4.40.0'
  pod 'FacebookLogin' '4.40.0'
  pod 'FacebookShare'
  pod 'ReachabilitySwift'
  </code></pre>
  
  <p>And got this error:</p>
  
  <pre><code>[!] Unable to satisfy the following requirements:
  
  - `FacebookCore (= 4.40.0)` required by `Podfile`
  
  None of your spec sources contain a spec satisfying the dependency: `FacebookCore (= 4.40.0)`.
  
  You have either:
   * out-of-date source repos which you can update with `pod repo update`.
   * mistyped the name or version.
   * not added the source repo that hosts the Podspec to your Podfile.
  
  </code></pre>
  
  <p>It's seem the gem's source is changed.</p>
  
  <p>Please help me.
  Thanks</p>