Work moar better on JRuby and Rubinius.

- Remove keyword arguments, as they're not properly supported yet.
- Use `Kernel::raise` wherever possible to avoid confusion.
- raise a `RuntimeError` in `Promise::Base#raise` when the value is not
an `Exception`.
gemfile.lock for jruby and rbx

added deps for jruby.
Disable EventMachine backend on JRuby.

Since @headius says it's dodgy, and that's my experience also.