Fix Gemfile, setting Active support to < 7.1.0 (#39828)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39828

Active Suppert released a new Gem which is incompatible with Cocoapods 1.13.0, the latest release, as they removed a method used by cocoapods.

This fix ensures that we install compatible versions of the Gem.

## Changelog:
[iOS][Fixed] - Set the max version of Active support to 7.0.8

Reviewed By: hoxyq

Differential Revision: D49949782

fbshipit-source-id: 278097502d3a416567cc8c0b90090fee4fb21503