add Mongoid support
enable Mongo on Travis
skip Mongoid specs if not present in the bundle
don't re-test Sequel and DataMapper support across Rails versions

Since these integrations have nothing to do with Rails, it's enough for
just the main Gemfile to include the libraries.
use "debugger" gem on 1.9
replace Rake + ruby CI runner with one in bash
add `each-gemfile` script