fix for CRM-13237 : we are currently not updating contribution activity while editing contributions, thats why the campaign id doesn't get saved in activity record during contribution edits

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* CRM-13237: Campaign ID in Activites table does not update after edits
  http://issues.civicrm.org/jira/browse/CRM-13237
CRM-13237 : code comment

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* CRM-13237: Campaign ID in Activites table does not update after edits
  http://issues.civicrm.org/jira/browse/CRM-13237
Merge pull request #1775 from pratik-joshi/CRM-13237

fix for CRM-13237 : we are currently not updating contribution activity while editing contributions, thats why the campaign id doesn't get saved in activity record during contribution edits