CRM-13685 - "Manage Extensions" - Fix upgrade check

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* CRM-13685: "Manage Extension" screen should use "<" to determine upgradeability
  http://issues.civicrm.org/jira/browse/CRM-13685
Merge pull request #1882 from totten/4.3-mgext-upg

CRM-13685 - "Manage Extensions" - Fix upgrade check