Now using pmpro_after_all_membership_level_changes action

[dparker1005]

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• [x] Have you followed the Contributing guideline?
• [x] Does your code follow the WordPress' coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Now using pmpro_after_all_membership_level_changes action instead of pmpro_after_change_membership_level. This allows for better compatibility with MMPU and 3rd party plugins that change membership level, such as WooCommerce.

Resolves #24.

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you successfully run tests with your changes locally?

Changelog entry

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