Replace "incorrect" types to make project generation work with xcodeproj

Created by: fdiaz

Buck currently outputs an XML for project.pbxproj where runOnlyForDeploymentPostprocessing and dstSubfolderSpec are treated as integer.

			<key>runOnlyForDeploymentPostprocessing</key>
			<integer>1</integer>
			<key>dstSubfolderSpec</key>
			<integer>16</integer>

When trying to open the .xcodeproj file with xcodeproj we're getting:

[Xcodeproj] Type checking error: got Fixnum for attribute: Attribute dstSubfolderSpec (type: simple, classes: [String], owner class: PBXCopyFilesBuildPhase)

This seems to be a bug in xcodeproj itself.

To work around this, I'm forcing these 2 keys to have Strings as values instead of Int. After doing this, xcodeproj can succesfully open our project.

This was all already done in our fork of buck here

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I'm also including here @donholly commit in their fork that adds the same workaround for the same issue in PBXContainerItemProxy https://github.com/Clubroom/buck/commit/8fa5ae58c5df0e991d2f2e80830a73206c384bf3