Hi kara,
I basically went through the same steps as you in order to update our plugin from JIRA 7.3.2 to JIRA 7.5.0.
Then I though about something which might seem weird, but what if the spring scanner just doesn’t see the interface because it is declared in a dependency (where the class AbstractWebCondition comes from).
So I just added implements interface Condition (com.atlassian.plugin.web.Condition) in my class and surprise! It works!
What I recommend to you is that you create a base class, lets call it BaseCustomCondition, for your custom conditions and declare that it extends AbstractWebCondition and that it implements the interface Condition.
Then, you can simply extends BaseCustomCondition instead of AbstractWebCondition and be sure that the scanner will see it.
Hope this helps.