Makes swig rendering synced

[oleggromov]

Trying to make my Swig theme working I've found a bug: server or generation process freezes on first page with content.

$ hexo generate --debug
[debug] 04:12:49.907 Configuration file load successfully
[debug] 04:12:49.915 Version info checked successfully
[debug] 04:12:49.916 Loading the database...
[debug] 04:12:50.190 Theme layout loaded
[debug] 04:12:50.220 Theme source loaded
[debug] 04:12:50.240 Theme configuration loaded
[debug] 04:12:50.264 Theme i18n loaded
[debug] 04:12:50.372 Processed: _posts/do-work-that-matters.md
[debug] 04:12:50.501 Cache saved
[info] 04:12:50.502 Files loaded in 0.346s
[debug] 04:12:50.507 Public folder cleared: css/style.css,archives/index.html,archives/2013/index.html,archives/2013/12/index.html,index.html,blog/do-work-that-matters/index.html
[debug] 04:12:50.507 Render archive: archives/
[debug] 04:12:50.508 Render archive: archives/2013/
[debug] 04:12:50.509 Render archive: archives/2013/12/
[debug] 04:12:50.509 Render index: 
[debug] 04:12:50.510 Render post: blog/do-work-that-matters/

I crawled a little around the source and found that swig renderer is registered as async.
Don't know if this is correct, but changing mode to sync fixes the issue and my site works finally!

[debug] 04:15:23.340 Render post: blog/do-work-that-matters/
[create] 04:15:23.420 Public: archives/2013/12/index.html
[create] 04:15:23.423 Public: blog/do-work-that-matters/index.html
[create] 04:15:23.493 Public: index.html
[create] 04:15:23.497 Public: archives/2013/index.html
[create] 04:15:23.512 Public: archives/index.html
[info] 04:15:23.513 5 files generated in 0.196s