Update podspec to support installation from CocoaPods

[naoey]

I have been able to successfully install and build this package from CocoaPods in my project using this podspec. This does imply that I have to install the iOSPhotoEditor depedency myself in my main Podfile, but I feel it is a much cleaner solution to have that stated in the install steps than the current setup.

Closes #47

I have been able to successfully use this in my Podfile on CocoaPods 1.6.1 and so far it has worked more reliably than the current install method.

pod 'RNPhotoEditor', :path => '../node_modules/react-native-photo-editor/ios'
pod 'iOSPhotoEditor', :git => 'https://github.com/prscX/photo-editor'

pre_install do |installer|
installer.analysis_result.specifications.each do |s|
  if s.name.include?('iOSPhotoEditor')
    s.swift_version = '4.1' unless s.swift_version
  end
end

Update

Additionally, I also got these libraries to work without the need to have use_frameworks! in the consumer's Podfile. If this method of installation is desirable by other users of this library I can update this PR with more details.

[prscX]

Thanks a lot @naoey for all the Effort you have added to this PR.

I have merged PR: 54 which also contains this PR changes. Considering that, I am closing this PR.

Thanks
</ Pranav >