Add SmartyPants test for trailing single quotes (see vmg/redcarpet#340)

examples:
- the students' chairs
- tryin' to fix this bug
Fix vmg/redcarpet#340 by replacing trailing single quotes
Add changelog entry for trailing single quotes (vmg/redcarpet#373)
Move changelog entry to the top of the file
Merge pull request #373 from kevin1/trailing_single_quotes

Convert trailing single quotes to curly quotes
Remove trailing whitespaces