Loosen phantom version. Allow 2.0

Since PhantomJS 2.0 is hopefully right around the corner, this adds support to poltergeist. I will likely be submitting additional PRs to directly support 2.0 (seeing a lot of test failures).

Fixes #535.
Even though a 3.x doesn't exist yet, it would be a good idea to guard against its accidental selection; when a user is using this release in the future (when phantomjs 3.x _is_ available), we'd rather not set them up for failure.

~~~ ruby
 PHANTOMJS_VERSION = ['>= 1.8.1', '< 3.0']
~~~