Set pre-release version

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
setup: try subprocess.check_output, failback to commands.getoutput

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
Bump version to 0.2.6

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>