iOS 6.1 causes crash on startup

Forgive me, I'm new to using raygun (and iOS for that matter), but are there any known bugs when using iOS 6.1? It works perfectly on iOS 7 and 8 but crashes the app on startup with iOS 6.1. I get the following in the debug console:

dyld: lazy symbol binding failed: Symbol not found: _OSAtomicIncrement32Barrier Referenced from: /var/mobile/Applications/Application ID/AppName.app/AppName Expected in: /usr/lib/libSystem.B.dylib

dyld: Symbol not found: _OSAtomicIncrement32Barrier Referenced from: /var/mobile/Applications/Application ID/AppName.app/AppName Expected in: /usr/lib/libSystem.B.dylib

The app runs fine when [Raygun sharedReporterWithApiKey:@"MY_KEY"]; is NOT called. I'm using Raygun4iOS Version 2.1.0. I haven't tried using any of the legacy versions; would that be a possible solution?