eat: get the proper MODVERSION

* The version in system/build.prop might be
  different than what the zip is named on incremental
  builds.
* Use CM_VERSION from the build system instead.

Change-Id: If4508921d8fd05219a2f358a2397a13b247065bd
Signed-off-by: Chirayu Desai <cdesai@cyanogenmod.org>