Fix check for parallel build support in Sphinx

The previous check was failing with any build version containing non-numeric parts, eg. 1.3a1.  This should work until Sphinx' 10.x release (which, given 1.0 took three years and we're not yet out of 1.x, seems unlikely in the near future.)

Signed-off-by: Sam Kleinman <samk@10gen.com>