renamed Notificaion to GCMPayload

[mludi]

naming conflict with Foundation Notification

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@eda75c9). Click here to learn what that means.
The diff coverage is 75%.

@@            Coverage Diff            @@
##             master       #2   +/-   ##
=========================================
  Coverage          ?   41.45%           
=========================================
  Files             ?        7           
  Lines             ?      193           
  Branches          ?        0           
=========================================
  Hits              ?       80           
  Misses            ?      113           
  Partials          ?        0

Impacted Files	Coverage Δ
Sources/VaporGCM/GCMPayload.swift	16.66% <ø> (ø)
Tests/VaporGCMTests/testVaporGCM.swift	80% <100%> (ø)
Sources/VaporGCM/PushMessage.swift	39.13% <50%> (ø)

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