Avoid testtools 0.9.25
    
    Fixes bug 1102400
    
    testtools 0.9.25 introduces a new dependency on the 'extras' package, but
    it does so in a way to prevents itself from being installed without the
    'extras' package already installed.
    
    Work around the problem for now by avoiding the problematic release.
    
    Change-Id: If8aa3925c744fad88f6f2715b5e779d0c258524c