Don't attempt native iOS login on pre-iOS 6 devices.

Calling [accountStore accountTypeWithAccountTypeIdentifier:@"com.apple.facebook"]
from the iPad 5.1 simulator causes the application to hang with this stack (tested
with JustRequestSample):

    frame #0: 0x91918c5e libsystem_kernel.dylib`semaphore_wait_trap + 10
    frame #1: 0x011b9022 libdispatch.dylib`_dispatch_semaphore_wait_slow + 108
    frame #2: 0x011b90fa libdispatch.dylib`dispatch_semaphore_wait + 36
    frame #3: 0x0011b35d Accounts`-[ACAccountStore accountTypeWithAccountTypeIdentifier:] + 199
    frame #4: 0x00014140 JustRequestSample`-[FBSession authorizeWithPermissions:defaultAudience:integratedAuth:FBAppAuth:safariAuth:fallback:isReauthorize:] + 529 at FBSession.m:893
    frame #5: 0x00013f27 JustRequestSample`-[FBSession authorizeWithPermissions:behavior:defaultAudience:isReauthorize:] + 118 at FBSession.m:839
    frame #6: 0x00012c22 JustRequestSample`-[FBSession openWithBehavior:completionHandler:] + 518 at FBSession.m:365
    frame #7: 0x000166e6 JustRequestSample`+[FBSession openActiveSessionWithPermissions:allowLoginUI:allowSystemAccount:isRead:defaultAudience:completionHandler:] + 270 at FBSession.m:1563
    frame #8: 0x000133a8 JustRequestSample`+[FBSession openActiveSessionWithReadPermissions:allowLoginUI:completionHandler:] + 82 at FBSession.m:533
    frame #9: 0x000028e1 JustRequestSample`-[JRViewController buttonRequestClickHandler:] + 289 at JRViewController.m:103

This makes it impossible to test apps in the 5.1 simulator.  Work around this by clearing tryIntegratedAuth on pre-6.x devices, where it's never supported anyway.

This isn't an issue on real 5.1 devices, but being able to test in the simulator for various versions is critical.

Tested on iPad 3 in iOS 5.1 and 6, and in the iOS 4.3, 5.1 and 6.0 simulators.