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Fix `python-M2Crypto` install issue on SLES 11 SP3

SLES 11 SP3 ships with both python-M2Crypto-0.22.* and python-m2crypto-0.21 and
we will be asked which we want to install, even with --non-interactive. Let's
try to install the higher version first and then the lower one in case of
failure.

Fixes #431.
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