Fix broken link to PHPExcel site

Summary:
phpexcel.net currently serves a 500 page, and the top Google hit for
PHPExcel (on codeplex) gives you a site warning you that it is 3 years
out of date, and to see GitHub instead.

Update the link from Maniphest's 'please install PHPExcel to enable
export' prompt.

Test Plan:

• Click 'Export to Excel' from Maniphest query result page without PHPExcel installed
• Follow suggested link to install it
• See code rather than a HTTP 500 error

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D16363