latest listen supports new wdm
Bundler advises not to use a source symbol anymore

- https://github.com/carlhuda/bundler/commit/d30026e9c8fc6c98478120866a47ca5b619251b8
Merge branch '3.0-stable' of github.com:middleman/middleman into 3.0-stable
adapt to bundler changes, add ruby 2.0 to tests, prep release