Remove of deprecated rspec-mocks methods

The "stub" syntax in rspec-mocks was replaced with "allow" syntax in
RSpec 3.
Eliminate warnings from using GitHub's preview API

If you don't explicitly add the `accept` option, Octokit prints a
warning to STDOUT every time you use part of the preview API.

There are no remaining warnings from GitHub after this change.
Use default show_exceptions value of false

Enabling this hides exceptions during integration tests behind an HTML
500 page, which makes debugging much harder. It can also hide errors, as
the test can render a 500 and still pass.

I didn't have redis running locally, which caused one test to fail. With
the previous setting, it took me 15 minutes to find the failure. With
the new setting, it immediately prints an error about redis.
Creat Plan class
Use new Plan class in Repo class
Use new Repo methods to access plan information
Remove plan information from Subscription class
Update fixture with new plan type
Update account page to support new plan type