Fixed support for iOS 6

The podspec for this project lists iOS 6 as the minimum version supported, however M13BadgeView calls `NSString`'s `- (CGRect)boundingRectWithSize:(CGSize)size options:(NSStringDrawingOptions)options attributes:(NSDictionary *)attributes context:(NSStringDrawingContext *)context`, which is an iOS 7 only method.

This pull request fixes support for iOS 6 by creating an `NSAttributedString` and using the iOS 6 method `- (CGRect)boundingRectWithSize:(CGSize)size options:(NSStringDrawingOptions)options context:(NSStringDrawingContext *)context`.

An alternate solution to this would be to drop support for iOS 6 by modifying the podspec.

(shout out to [Deploymate](http://www.deploymateapp.com) for helping me pinpoint this issue).