Fix issue with minor version of CURRENT_VERSION_ID

Fix issue with minor version of CURRENT_VERSION_ID as described in https://github.com/gae-init/gae-init/issues/90 that caused exceptions (note: sometimes, not always... due to randomness of the specific value assigned to it on the development server) when converting into CURRENT_VERSION_DATE.

This PR suggests to drop the CURRENT_VERSION_TIMESTAMP as this doesn't seem to be used anywhere else in code or templates.