simplify previous_sha logic for github case (#70)

Author: NicoHinderling

lib/utils/github.rb

@@ -50,7 +50,8 @@ def self.repo_name
     end
 
     def self.previous_sha
-      Git.previous_sha
+      return unless push?
+      github_event_data[:before]
     end
 
     def self.github_event_data