pod 'AFNetworking', '~> 1.0' 版本號可以是1.0，可以是1.1沿腰，1.9览徒，但必須小于2
－個更簡單的podfile:
pod 'AFNetworking', '1.0' // 版本號指定為1.0
一個更更簡單的podfile:
pod 'AFNetworking',? // 不指定版本號，任何版本都可以
一個復(fù)雜的podfile:
Besides no version, or a specific one, it is also possible to use logical operators:
'> 0.1'Any version higher than 0.1 0.1以上
'>= 0.1'Version 0.1 and any higher version 0.1以上颂龙，包括0.1
'< 0.1'Any version lower than 0.1 0.1以下
'<= 0.1'Version 0.1 and any lower version 0.1以下习蓬，包括0.1
In addition to the logic operators CocoaPods has an optimisic operator~>:
'~> 0.1.2'Version 0.1.2 and the versions up to 0.2, not including 0.2 and higher 0.2以下(不含0.2)纽什，0.1.2以上（含0.1.2）
'~> 0.1'Version 0.1 and the versions up to 1.0, not including 1.0 and higher 1.0以下(不含1.0)，0.1以上（含0.1）
'~> 0'Version 0 and higher, this is basically the same as not having it. 0和以上友雳，等于沒有此約束
