fix missing groups

Hi,

Since this plugin hasnt been updated for a while, here is how to fix it.

1. download it
2. download latest php sdk from mailerlite
https://github.com/mailerlite/mailerlite-api-v2-php-sdk
https://bit.ly/32jmi7M
3. unpack plugin from step 1
4. replace vendor folder with the one from new Mailerlite sdk
5. zip it up
6. install plugin via Upload button in WP Dashboard
7. It works!