Removed shell_exec instances for md5 checksums of zip files, now uses only php's md5_file function, beacause  on the majority of php installations, shell_exec is disabled for security reasons.

I suggest to use the timestamp contained in the zip file (ro.build.date.utc) in place of the zip file date, because this can cause troubles in some circumstance, for example:

If you upload a file by hand on the server, the creation date of the file will be newer than the compillation date of the ROM and CMUpdater will believe that is a new version because CMUpdater compare the date contained in the build.prop (ro.build.date.utc).