avisynth: Fix detection of AviSynth 2.5

In order to safely exit when the user tries to use AviSynth 2.5, the
continue_on_fail value for 2.6's functions need to be set to 1.
Otherwise, the library loader fails before the 'upgrade to 2.6'
log message appears.

Signed-off-by: Diego Biurrun <diego@biurrun.de>