Bug 931718 - Append 'xulrunner-sdk' to XRE Path

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, and we need
to append 'xulrunner-sdk' so that the binary can be used.
Merge pull request #292 from lissyx/bug931718

Bug 931718 - Append 'xulrunner-sdk' to XRE Path r=ahal