Fix access URL for sub job in MultiJobBuild view

When hudson is not at the top path of the webserver (ie htt://server/hudson/), the link to the sub job was broken.
Use ${rootURL} to fix this

Signed-off-by: Nicolas Morey-Chaisemartin <nmorey@kalray.eu>
Merge pull request #44 from nmorey/bad-subjob-url

Fix access URL for sub job in MultiJobBuild view