Bug 1098406: Fix gaia repository path.

A commit in the mozilla-central repository [1] broke the gaia URL
repository URL construction code.

We now make sure our code works independent of the leading slash
presence.

[1] http://hg.mozilla.org/mozilla-central/rev/f3782f5c2710
Merge pull request #38 from walac/bugz/1098406

Bug 1098406: Fix gaia repository path.