specify newer version of virtus in gemspec

use version 1.0.2, since older versions have incompatible api

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
Look up story without project

- Changed Endpoints::Story.get to take only story_id
- Removed all endpoints except Story because they have inconsistent and
  incomplete test coverage and we don't need them.
- Use webmock instead of VCR for testing

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>