update .podspec, remove -ObjC and -all_load flags

[aletan]

Fix conflict with other pods (example libjingle_peerconnection)

[boxcla]

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[aletan]

CLA signed

[boxcla]

Verified that @aletan has just signed the CLA. Thanks, and we look forward to your contribution.

[RicoYao]

@aletan Thanks for submitting this.

I believe the reason we added all_load was to have the categories compiled in. Without it, one might run into runtime errors (crashes). We should test that category methods still work with this, and also look into whether we need to use force_load instead of all_load.

[boxtcarpel]

We have moved to using Carthage. Thank you for your pull request and for using the SDK.