Fresh install will cause warning in Plugin page

[doekenorg]

When the migration runner is instantiated, it looks for the migrations in the src directory, directly in the plugin folder. But since we now use Strauss with a src relocation, that folder doesn't exist. So it throws a warning.

• Add a is_readable check
• Check if we are in a build state, and change the path accordingly. (See AddOnProvider).

[bennemann]

https://secure.helpscout.net/conversation/2764130229/58986/