Better handling of unavailable shops for RecurringApplicationCharge

The current implementation will throw a NoMethodError: undefined method
`find' for nil:NilClass exception when calling
ShopifyAPI::RecurringApplicationCharge.current,
ShopifyAPI::RecurringApplicationCharge.pending, etc on an uninstalled or
otherwise unavailable shop.

This improvement will simply return a nil, which is consistent with
ShopifyAPI::RecurringApplicationCharge.all for the uninstalled shop
Update admin link in API console
Updating tests
Oops, let's fix that test
Fixed test again
Test for RecurringApplicationCharge 404 response

test case for https://github.com/Shopify/shopify_api/pull/147
Merge pull request #148 from Shopify/change_admin_link

Update admin link in API console
Merge pull request #147 from ShopStorm/master

Better handling of unavailable shops for RecurringApplicationCharge
Minor doc fixes
Merge pull request #152 from ukd1/master

Minor doc fixes
rdoc -> md
Minor tweaks
Merge pull request #153 from ukd1/master

Migrate rdoc -> markdown
Update README.md

just typo and copyright year
Merge pull request #154 from mtmail/patch-1

Update README.md
Fixing Gemspec to point to correct README
Merge pull request #155 from joncalhoun/patch-1

Fixing Gemspec to point to correct README
Update tests to use search endpoint
Use customer search endpoint
Specify minitest 4.2