Use constant for repository URL

This will eventually be referred to in multiple places, so use a
constant for ease of reuse.

Change-Id: Ibb3a8ec10a777a8f5dab7817d7ddc47d3e8f16d8
Reviewed-on: http://gerrit.causes.com/36172
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>