Detect previously migrated permission scheme

JIRA 6.2.5 used to verify.

Steps to reproduce:

Create a project in JIRA
Install JSD 1.2.5 and enable JSD for the project
Migrate the permission scheme
Upgrade JSD to 1.2.6.1

Expected:

Permission scheme was already previously migrated, nothing to do.

Actual:

You are asked to migrate the permission scheme again