Bug 931718 - Fix XULRunner SDK path detection

With bug 906316 we are now able to have multiple versions of the SDKs on
disk, to avoid always re-downloading them. Path has changed, so we made
Gaia able to say us where it is thanks to the print-xulrunner-sdk make
target. Now, let's use it on our side.
Merge pull request #294 from lissyx/bug931718-v2

Bug 931718 - Fix XULRunner SDK path detection r=ahal