Fix license warning with subspecs

[kylefleming]

CocoaPods-binary removes the license attribute from the podspec attributes hash since it doesn't copy the license file over. However, the license attribute in only valid in the root spec of a podspec.

There is currently a bug where if you specify a dependency on a subspec of a pod, then cocoapods-binary removes the license attribute not from the root spec, but from the subspec, thus leaving the license attribute in the root spec intact. This produces the following warning:

[!] Unable to read the license file `LICENSE` for the spec `<PodName> (<PodVersion>)`

This PR fixes this bug by using the root spec when removing the license attribute.

[anton-plebanovich]

Faced with the same issue. It'd be awesome to merge this one.

[anton-plebanovich]

I was able to reproduce with pod 'Moya/Core', :binary => true