Use PHPExcel, not Spreadsheet_Excel_Writer, to export Excel sheets

Summary: Sandra had trouble opening the Spreadsheet_Excel_Writer ones so use
PHPExcel, which is way better, just a bit more complicated.

Test Plan:

• Generated modern Excel 2007 .xslx sheets.
• Opened them in Excel in Office Mac 2011.
• Opened them in Apple Numbers from the app store.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran, epriestley

Maniphest Tasks: T911

Differential Revision: https://secure.phabricator.com/D1744