Fix compile warnings

[kessiler]

Would be great to fix these warnings..

UniversalAnalyticsPlugin.m:173:50: warning: incompatible pointer to integer conversion sending 'NSNumber *__strong' to parameter of type 'NSUInteger' (aka 'unsigned int') [-Wint-conversion]
    [tracker set:[GAIFields customMetricForIndex:key]
                                                 ^~~
UniversalAnalyticsPlugin.m:7:GAIFields.h:131:48: note: passing argument to parameter 'index' here
+ (NSString *)customMetricForIndex:(NSUInteger)index;
                                               ^
UniversalAnalyticsPlugin.m:174:43: warning: incompatible pointer to integer conversion sending 'NSNumber *__strong' to parameter of type 'int' [-Wint-conversion]
           value:[[NSNumber numberWithInt:value] stringValue]];
                                          ^~~~~
In module 'Foundation' imported from UniversalAnalyticsPlugin.h:4:
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator9.3.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSValue.h:91:34: note: passing argument to parameter 'value' here
+ (NSNumber *)numberWithInt:(int)value;
                                 ^
2 warnings generated.

Plugin version: 1.5.6

[victorsosa]

PR welcome