Remove use of open-uri and dependency on vcr

#4 moved the `require 'open-uri'` from DiskStore into the specs so `open-uri` would not be loaded in production.  It would be better to remove the use of `open-uri` entirely, and this has a nice side effect of removing a number of development dependencies.

Only one spec uses `open-uri`, and it does so with this URL:

    $ irb
    2.0.0-p353 :001 > require 'open-uri'
    => true 
    2.0.0-p353 :002 > open("http://media.giphy.com/media/KXD1pSzGb3Khi/giphy.gif")
    => #<Tempfile:/var/folders/86/_2n9z60n7f7d9r3x11f4b3340000gn/T/open-uri20140526-6669-41ytgw> 

The result is a `Tempfile` with binary data.  

The other specs already use a `Tempfile` but do not test binary data.  This change removes the spec depending on `open-uri` and changes the others to use binary data.