OC创建Podfile的时候，官网推荐用这种格式使用，
platform :ios, '8.0'
use_frameworks!
target 'ProjectName' do
pod 'AFNetworking', '~> 2.6'
pod 'MJExtension', '~> 2.3'
end
Swift创建Podfile
source 'https://github.com/CocoaPods/Specs.git'
platform :ios, "8.0"
use_frameworks! #swift版本要加这句
target :'ProjectName' do
pod 'AFNetworking'
pod 'SVProgressHUD'
pod 'SDWebImage', '~> 4.0.0'
pod 'SnapKit'
end
