Peg simplecov version to 0.8.0.pre

simplecov >= 0.8.1 causes an issue preventing execution of the
Test::Unit suite.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #1066 from opf/fix/peg-simplecov-version

Peg simplecov version to 0.8.0.pre