Restrict personal namespace PPA to precise

My goal is to be able to make precise/mediawiki charm usable on trusty/arm64
until a proper trusty/mediawiki charm exists.

Today this charm immediately fails when deployed on an architecture that is not
supported by the ppa:clint-fewbar/fixes. This PPA usage is required to go away
before promotion to trusty (LP: #1298674), so let's just restrict the usage
of this PPA to precise hosts for now.

Tested on ec2 in precise on verified that the ppa is still added.
Tested on local provider trusty/arm64 and verified that the ppa is no longer
added and the install hook proceeds.
[dannf] Restrict personal namespace PPA to precise