Reject Podfiles which redefine the platform within the same block

[mrackwitz]

Podfile which redefine the platform within the same block in a Podfile should be rejected as invalid. e.g.

platform :ios, '8.0'
pod 'SDWebImage'
platform :ios, '6.1'
pod 'AFNetworking', '~> 3.0'

[segiddins]

Why is this desirable?

[billychen2014]

hmm, two questions about such fix.

1. how to deal with the situation that the platform only can be declared once per block ?As SDWebImage need ios 6.1 but af need 8.0.
2. IS SDWebImage can work or not because of the platform overwrites?

I did such testing:

1. tried to declare target twice (i.e.: two blocks) to match the different platform requirement, result is
   [!] The target testSDWeb is declared twice.
   2, test the SDWebImage function ,it looks like it works well, so. it looks like there are some more complex things .