Updated to 5.2

[finepointcgi]

• Updated gradle version
• updated the billing verison
• Addresses (in theory) #43 #35

[finepointcgi]

This PR is if we dont want to break backwards compatibility with other versions of this plugin. We will need to use #36 if we want to move to 6.0. Branching off of his work for that migration.

[akien-mga]

For the record, your commit seems not to be linked to your GitHub account. See: Why are my commits linked to the wrong user? for more info.

[timoschwarzer]

Still can't test, but the code looks good to me

[akien-mga]

Thanks!