Update Tweet Max Length to 280

[Alqueraf]

Hi, thanks for maintaining the fork!

Could you update the tweet max length to the new 280 limit instead of 140?

twitter-kit-ios/TwitterKit/TwitterKit/Social/Syndication/ThirdParty/TWTRTwitterText.m

Line 243 in ee9a81e

static const NSUInteger MaxTweetLength = 140;

Thanks