这是我的播客文件：

source 'https://github.com/CocoaPods/Specs'
source 'https://github.com/CocoaPods/Specs.git'
platform :ios, '8.0'

target 'Unity-iPhone' do
pod 'Google-Mobile-Ads-SDK', '7.3.1'
pod 'Firebase/Core'
end

当我运行“pod install”时，它工作得很好
输出：

Analyzing dependencies
Downloading dependencies
Using Firebase (4.13.0)
Using FirebaseAnalytics (4.2.0)
Using FirebaseCore (4.0.20)
Using FirebaseInstanceID (2.0.10)
Using Google-Mobile-Ads-SDK (7.3.1)
Using GoogleToolboxForMac (2.1.4)
Using nanopb (0.3.901)
Generating Pods project
Integrating client project
Sending stats
Pod installation complete! There are 2 dependencies from the Podfile and 7 total pods installed.

完美，一切都很好，但如果我添加一个特定的版本与FirebaseCore，像下面的代码：

pod 'Firebase/Core','4.0.20'

和“pod安装”
输出：

[!] CocoaPods could not find compatible versions for pod "Firebase/Core":
  In Podfile:
    Firebase/Core (= 4.0.20)

None of your spec sources contain a spec satisfying the dependency: `Firebase/Core (= 4.0.20)`.

You have either:
 * out-of-date source repos which you can update with `pod repo update` or with `pod install --repo-update`.
 * mistyped the name or version.
 * not added the source repo that hosts the Podspec to your Podfile.

Note: as of CocoaPods 1.0, `pod repo update` does not happen on `pod install` by default.

实际上，FirebaseCore与4.0.20是存在的，那么我应该如何修复它？