Cocoapods install IGListkit slow

I found that IGListKit is always the lowest pod of our project.
So I use 'pod install --verbose' to find out why.
```
-> Installing IGListKit (3.1.1)
$ /usr/bin/git ls-remote https://github.com/Instagram/IGListKit.git stable
cce5a462bb8e4e3cf5b4d0aeebfedb318e52cf09 refs/heads/stable
> Copying IGListKit from `/Users/Shared/Jenkins/Library/Caches/CocoaPods/Pods/Release/IGListKit/3.1.1-cb97f` to `Pods/IGListKit`

```

I think its no need to git ls-remote, as we all know github.com is not fast as US.
Then I check the podspec of IGListKit, maybe is " :branch => 'stable' " let pod do this.
```
s.source = {
:git => 'https://github.com/Instagram/IGListKit.git',
:tag => s.version.to_s,
:branch => 'stable'
}
```