Only invoke TestFlight.takeOff when app launches instead of every time when app becomes active

According to https://testflightapp.com/sdk/ios/doc/2.0.0/, 

+[TestFlight takeOff:] should be called from -application:didFinishLaunchingWithOptions: and 

> Sessions automatically start at app launch, app did become active, and app will enter foreground and end at app will resign active, app did enter background, or app will terminate.

implies that the SDK already registers observers for UIApplication state changes to handle session end and resume.

Also related to #18