remove Ruby 1.8 support

The wikicloth dependencies httparty and nokogiri have required Ruby 1.9+
for a while now. Remove Ruby 1.8 support from wikicloth too.
travis: update Ruby versions

Remove Ruby 1.8.7, since some of the dependencies (httparty and
nokogiri) require Ruby 1.9.3.

Add Ruby 2.0 and 2.1 to the Travis config.