Move Dropbox-Sync-API-SDK 2.0.0-b3 podspec to the correct library

A 2.0.0-b3 version of the Dropbox-Sync-API-SDK was inadvertently created in the Dropbox-iOS-SDK library. This fixes the podspec
and moves it to the correct Dropbox-Sync-API-SDK library.
Merge pull request #3154 from daikini/master

Move Dropbox-Sync-API-SDK 2.0.0-b3 podspec to the correct library