Replace native serialize function with abstract serializer

[Detzler]

Fixes #23

Instead using native serialize function, the fix implements the usage of the serializer interface. I've tested at a real project. Works fine.

This fix does not include backward compatibility which would look like:

1. On module upgrade get the value via unserialize function
2. save the value via the serializer interface

Steps to install the fixed module:

1. Delete the module from the database DELETE FROMsetup_moduleWHEREsetup_module.module= 'Magenerds_BasePrice'
2. Delete cache php bin/magento cache:clean
3. Install module php bin/magento setup:upgrade

[Detzler]

Oh, there is already a fix lol never mind