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