Remove old zip file before making zip file.

If old zip file exist, zip.rb throw Zip::ZipEntryExistsError. Therefore CrxMake.zip cannot make new zip file.
In order to solve this problem, add remove old zip file function, before making it.

Remove zip file function is already inpremented in code.(function name = final) But, function name 'final' is not suitable for not final place.
So, I rename final to remove_zip.
Merge pull request #10 from samurai20000/zipexisterror

Remove old zip file before making zip file.