Cocoapods 1.3.1 release (#29240)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29240
### Summary
The 1.3.1 binary has been uploaded to AWS - https://ossci-ios.s3.amazonaws.com/libtorch_ios_1.3.1.zip. This PR updates the cocoapods version to 1.3.1
### Test Plan
- The 1.3.1 binary works well
Test Plan: Imported from OSS
Differential Revision: D18333750
Pulled By: xta0
fbshipit-source-id: fe6e42c51f3902ad42cab33f473dffb0f6f33333
