Remove Gemfile.lock from the repo per community recommendations

Reference: http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/
Tighten up some dependencies
Update Changelog.md
Support :nth-child, etc. selectors [fixes #49]

This change will have warnings emitted when generating email depending
on the chosen rules. Previously Roadie would silently skip all rules
that contained a ":", which would skip working selectors as well.

This change tries all such selectors (unless they are inside the
blacklist) and warn on any errors. I expect the blacklist to be expanded
when users start reporting these warnings, and perhaps even discover
problems they didn't knew they had in their stylesheets.