CRM-14894 - Modify line_item unique index in schema line_item.xml to fix fatal for membership renewals.

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* CRM-14894:
  https://issues.civicrm.org/jira/browse/CRM-14894
CRM-14894 - Modify line_item unique index in schema line_item.xml to fix fatal for membership renewals.

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* CRM-14894:
  https://issues.civicrm.org/jira/browse/CRM-14894
Merge pull request #3551 from davecivicrm/CRM-14894

Crm 14894