fix precision conversion warning

This silents the warning
```
YTVimeoExtractor/YTVimeoExtractor/YTVimeoExtractor.m:110:92: Implicit conversion loses integer precision: 'long long' to 'NSUInteger' (aka 'unsigned int')
```

on xcode 5

YTVimeoExtractor/YTVimeoExtractor.m

@@ -107,7 +107,7 @@ -(void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLRespons
         [connection cancel];
     }
 
-    NSUInteger capacity = (response.expectedContentLength != NSURLResponseUnknownLength) ? response.expectedContentLength : 0;
+    NSUInteger capacity = (response.expectedContentLength != NSURLResponseUnknownLength) ? (uint)response.expectedContentLength : 0;
     self.buffer = [[NSMutableData alloc] initWithCapacity:capacity];
 }