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