Upgrade to Jasmine 2 and add Rake-task `jasmine:ci`

Summary:

* upgrade to Jasmine 2
* add Rake-task `jasmine:ci` that boots Middleman server and runs tests with PhantomJS
* describe in README.md + improve formatting a bit
* update CHANGELOG.md
* bump version

The Rake-task was taken from https://github.com/pivotal/jasmine-gem/blob/master/lib/jasmine/tasks/jasmine.rake and then I removed everything, except the `jasmine:ci`-task which I modified a bit to work with Middleman.

NOTE: There's a FIXME in the Rake-task. The url is hardcoded to `/jasmine`. I would appreciate help with fixing that if it's important.