feat: update minimum php-imap version for ^2.0.4

[Chris53897]

https://github.com/secit-pl/imap-bundle/issues/19

I did give a wrong answer in the linked issue.

2.0 to 2.0.3 of php-imap is missing the required ext-imap: * https://packagist.org/packages/php-imap/php-imap#2.0.3

I find this out, after i noticed that ext-imap was not installed at my local machine.