Support testing on iPhone 6+ with @3x assets

Without this pull request try to run unit tests from the demo project in Xcode 6 on iPhone 6 Plus.

Here is what I was getting:

```
Test Suite 'FBSnapshotTestCaseDemoTests' started at 2014-09-26 18:44:07 +0000
Test Case '-[FBSnapshotTestCaseDemoTests testExample]' started.
Sep 26 21:44:08 Victors-MacBook.local FBSnapshotTestCaseDemo[63606] <Error>: CGBitmapContextCreate: invalid data bytes/row: should be at least 768 for 8 integer bits/component, 3 components, kCGImageAlphaPremultipliedLast.
Sep 26 21:44:08 Victors-MacBook.local FBSnapshotTestCaseDemo[63606] <Error>: CGContextScaleCTM: invalid context 0x0. This is a serious error. This application, or a library it uses, is using an invalid context  and is thereby contributing to an overall degradation of system stability and reliability. This notice is a courtesy: please fix this problem. It will become a fatal error in an upcoming update.
2014-09-26 21:44:08.359 FBSnapshotTestCaseDemo[63606:1686243] If you have Kaleidoscope installed you can run this command to see an image diff:
ksdiff "/Users/yas/Library/Developer/CoreSimulator/Devices/C4112E4C-5E08-44BB-8FAD-F1BEC7574F2C/data/Containers/Data/Application/74848DC3-1A64-4675-87D7-26E929ECD807/tmp/FBSnapshotTestCaseDemoTests/reference_testExample@2x.png" "/Users/yas/Library/Developer/CoreSimulator/Devices/C4112E4C-5E08-44BB-8FAD-F1BEC7574F2C/data/Containers/Data/Application/74848DC3-1A64-4675-87D7-26E929ECD807/tmp/FBSnapshotTestCaseDemoTests/failed_testExample@2x.png"
/Users/yas/code/ios-snapshot-test-case/FBSnapshotTestCaseDemo/FBSnapshotTestCaseDemoTests/FBSnapshotTestCaseDemoTests.m:32: error: -[FBSnapshotTestCaseDemoTests testExample] : ((comparisonSuccess__) is true) failed - Snapshot comparison failed: Error Domain=FBSnapshotTestControllerErrorDomain Code=4 "Images different" UserInfo=0x7ff669728fa0 {NSLocalizedDescription=Images different}
```

As iPhone 6 Plus uses `@3x` it seems reasonable to start to support this :)