CI: Cache installed pods

[mkchoi212]

Checklist

• I've run bundle exec fastlane test from the root directory to see all new and existing tests pass
• I've followed the vlc-ios code style and run bundle exec fastlane lint to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Description

According to CircleCI,

If you are using CocoaPods, then we recommend that you check your Pods directory into source control. This will ensure that you have a deterministic, reproducible build.
https://circleci.com/docs/2.0/testing-ios/#using-cocoapods

But Pods is 2.6 GB while the entire project is 2.8 GB 😅
So this PR tries to cache the pods in CircleCI instead of the repo with contents of Podfile as the key to the cache.

[carolanitz]

this was merged with 9573d3a