Fix lack of single curly quotes in 'markdown' filter.
Fix lack of single curly quotes in 'markdown' filter.

This is needed because of a bug in vmg/redcarpet#57. There is a test
assertion in test_helper to verify that this is actually necessary, and
the condition means it will automatically turn off should redcarpet have
this bug fixed for the user.

aprescott/serif,aprescott/serif