Your problem is that support for the Nexus 7 (gen 1) has been dropped, and this method won't work any more. This was announced on 03-Mar-2014 (as noted below).
In formal terms:
Support (as reference hardware) for the Nexus 7 (original model) is now deprecated, with development now focused on: Nexus 4 smartphone, Nexus 7 (2013) tablet, and Nexus 10 tablet.
Supported devices and codenames
The table below lists the supported devices and their corresponding factory images, ..
Nexus Device Support Dropped
Along with no dice for the Nexus 5, moving Ubuntu Touch to an Android 4.4 enablement stack has resulted in further decisions on which devices should remain “officially supported”.
From the end of January official builds for three of the four currently supported Nexus devices will be discontinued, affecting users of:
- Nexus 10 (2012)
- Nexus 7 (2012)
- Galaxy Nexus
This article does not exactly match current support, as Nexus 10 in still (for the moment) being supported. However, focus (going forward) is definitely on the Nexus 4 (for phone), and Nexus 7 (2013) (for tablet).