update to rspec3

Updates tests to Rspec version 3.  I also wanted to get rid of this warning:
```
DEPRECATION WARNING: In active-fedora 8 the solr fields created by Hydra::Datastream::Properties will be prefixed with "properties__".  If you want to maintain the existing behavior, you must override Hydra::Datastream::Properties.#prefix to return an empty string. (called from to_solr at /Users/awead/.gem/ruby/2.1.1/gems/active-fedora-7.1.0/lib/active_fedora/om_datastream.rb:51)
```
But that would require an update to Hydra.  So I'm leaving that one warning in there for now.