Minor fixes

[ob-stripe]

A bunch of minor fixes:

• Remove deprecated exception constructors
• Make Dispute.getIsChargeRefundable() return a Boolean
• Remove deserializers from StripeObject.PRETTY_PRINT_GSON
  The correct Gson instance to use when deserializing resources is ApiResource.GSON, this one is used only to serialize resources via toString() or toJson().
• Remove TODOs about subResourceUrl
  Unfortunately, it's not really possible to use subResourceUrl everywhere because the URLs for nested resources don't map 1:1 with the resource names, e.g. the URL for application fee refunds is /v1/application_fees/:id/refunds and not /v1/application_fees/:id/fee_refunds.
• Remove deprecated TransferReversalCollection.retrieve(String id, String apiKey) method (missed this one in Remove deprecated methods #532)
• Remove some unused imports and methods
• Use lombok's @Cleanup annotation to close resources without having to use try/finally

r? @brandur-stripe @remi-stripe
cc @stripe/api-libraries

[remi-stripe]

Damn the Cleanup thing is pretty neat