add Rails 4 to compatibilty in README
drop 1.8.7, ree and 1.9.2 from travis
minimum Ruby 1.9.3
no need to be strict on nokogiri version now that we're on 1.9.3 only
Merge branch 'master' into require-ruby-193

Conflicts:
	README.textile
	formtastic.gemspec
Merge pull request #949 from justinfrench/require-ruby-193

Require Ruby >= 1.9.3