Install appraisals before running the test suite

Currently, `rake` does run the `appraise` task before running tests. If
you run `rake appraise` on its own, it runs `rake appraise:install`
beforehand. However, the way that `rake` runs `appraise` is by
executing, not invoking, it. Hence, `appraisal:install` will actually
not be run (as #execute does not run dependencies for the task being
executed).

This fix is mostly useful when running tests locally -- on Travis this
isn't a problem b/c we are running `rake spec cucumber` instead of just
`rake`.
Use the new appraisal exec in 1.0.0.beta1
Update CONTRIBUTING with proper instructions for running tests