Fix build issues

Fix a number of build issues:

* Commit 09178badc34810722e0bc1956adca5241c3a08d0 used `CURRENT_PRODUCT_VERSION` in the kext plist file.  This should have been `CURRENT_PROJECT_VERSION`.  Replace this with `MODULE_VERSION` and define `MODULE_VERSION` as `CURRENT_PROJECT_VERSION`.
* Older versions of Xcode do not support C++11.  Replace `nullptr` with `NULL`.
* Update `.gitignore` to hide Xcode generated files.