Fixed crash due to the referrer dictionary being assign

`assign` is a synonym for `unsafe_unretained` in ARC - this is bad!

Analytics/Classes/Internal/SEGSegmentIntegration.m

@@ -72,7 +72,7 @@ @interface SEGSegmentIntegration ()
 @property (nonatomic, strong) NSMutableDictionary *traits;
 @property (nonatomic, assign) SEGAnalytics *analytics;
 @property (nonatomic, assign) SEGAnalyticsConfiguration *configuration;
-@property (nonatomic, assign) NSDictionary *referrer;
+@property (nonatomic, copy) NSDictionary *referrer;
 @property (nonatomic, copy) NSString *userId;
 @property (nonatomic, strong) NSURL *apiURL;
 @property (nonatomic, strong) SEGHTTPClient *httpClient;