feature: add installationSource to identify test versions and sideloaded apps

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This additional exported information can tell how the app came to be installed on the device. We use it to differentiate between App Store / Play Store versions and their TestFlight / Hockeyapp counterparts.

Example code would look something like this:

const isTestFlightOrSimulator =
  !!installationSource &&
  (installationSource.indexOf('sandboxReceipt') !== -1 ||
    installationSource.indexOf('CoreSimulator') !== -1)

const isAndroidBeta =
  !installationSource ||
  installationSource.indexOf('com.android.vending') === -1