Have sync captions use the new --english-caption-dir

This will allow sync captions to lower the cutoff limit to being close
to what it is for any English transcript that has made it to
published_prod.

Test Plan:
tools/khantube.py ../webapp-i18n-data/captions/incoming/ ../webapp-i18n-data/captions/published/ --data-file=../webapp-i18n-data/captions/amara_progress.json --english-caption-dir=../webapp-i18n-data/captions/published_prod/

Saw:
[Wed Oct 01 20:50:40 2014]    INFO : English subtitles are 0.995 complete
[Wed Oct 01 20:50:40 2014]    INFO : Got captions file ../webapp-i18n-data/captions/incoming/mr/L82bDTBMGUU.srt
[Wed Oct 01 20:50:40 2014]   ERROR : The video is only 0.863 % complete, but we have a cutoff at 0.945
Which is the correct lower cutoff for that particular video.

Auditors: csilvers