fix deprecated method call warning on iOS7

...Pods/MNMBottomPullToRefresh/MNMBottomPullToRefreshDemo/MNMBottomPullToRefresh/MNMBottomPullToRefreshView.m:156:48: warning: 'sizeWithFont:' is deprecated: first deprecated in iOS 7.0 - Use -sizeWithAttributes: [-Wdeprecated-declarations]
    CGSize messageSize = [[messageLabel_ text] sizeWithFont:[messageLabel_ font]];
                                               ^
In module 'UIKit' imported from …/Pods/Pods-MNMBottomPullToRefresh-prefix.pch:2:
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator7.0.sdk/System/Library/Frameworks/UIKit.framework/Headers/UIStringDrawing.h:59:1: note: method 'sizeWithFont:' declared here
- (CGSize)sizeWithFont:(UIFont *)font NS_DEPRECATED_IOS(2_0, 7_0, "Use -sizeWithAttributes:");
^
1 warning generated.