Upgrading Transporter Desktop.app to v2.8.1

The older 2.6.10 version would attempt to auto-update but has a bug
on Yosemite which would result in an endless loop and require a Force
Quit.
Merge pull request #7043 from cclark/transporter-desktop-v2.8.1

Upgrading Transporter Desktop.app to v2.8.1