Use jsdom rather than domino

domino doesn't support the full DOM API; in particular,
the "elements" property on form elements is missing.

jsdom does a better job of supporting the full DOM API, but
requires an older version of node than I was using. So I changed
the node engine requirements in package.json and .travis.yml
to ensure that tests are run with the correct node version.
Add test for serialize
Fix removeClass tests to work with jsdom

In domino, $(selector).attr('attr') returns undefined if
an attribute is not defined. In jsdom, the same method call
returns null (which matches the browser more closely).

Fix the tests so they pass with jsdom.