Fix some issues in v1.4.1

I am upgrading Soomla project for our games, and found some building issues in v1.4.1.

1. There are compile errors in VirtualItem.cs due to undefined symbol "DllImport", which is defined under System.Runtime.InteropServices

2. the mod_pbxproj.pyc is missing, so the post-build process on iOS will fail. Developers must add the three necessary libraries by themselves.

3. The event dispatcher on iOS send "onRestoreTransactions" to Unity which raise an error.  It should be "onRestoreTransactionsFinished" now.