Fixes xcode12 bolts compile error

[theextremeprogrammer]

First of all - thanks to the maintainers for your work on this project! It was a relief (and still is) to not have to migrate off of Parse.

Contribution guidelines
[x] 1. Fork the repo and create your branch from master.
[x] 2. Add unit tests for any new code you add. → No new code added, so I did not add any tests.
[x] 3. If you've changed APIs, update the documentation and the iOS Guide
[x] 4. Ensure the test suite passes. → Tests around extensions failed both with and without my changes; these do not seem to be related to this change.
[x] 5. Make sure your code follows the style guide

This PR is a simple fix for issue #1545 based on the discussion that was there.

When I compiled the code locally it indicated an import error for @import Bolts however Xcode still let everything compile. It was only once I had this imported in my application that the error caused my app to not compile.

Taking a look at the code, it seemed that anything that was using ParseInternal.h needed to use system-level #import <filename.h> as opposed to module-level imports @import module. This was the only module-level import in that hierarchy of file imports.

Thanks to @idefen1 for providing the original suggestion for the fix!

Please let me know if there's anything else that needs to be adjusted.

[drdaz]

Thanks for this!