Update webhooks API (event types and versioning)

[rsedykh]

New event types. Current enum:

• file.uploaded
• file.infected (it will be deprecated in favor of info_upldated in the future updates)
• file.stored
• file.deleted
• file.info_updated

https://uploadcare.com/docs/webhooks/#event-types

Also, please check the versioning. It's easier to just send current API version, because from within API 0.7 you can't create 0.6 anyways (but you'll be able to create 0.8 when they it is released).

Payload is also updated (new initiator field), but since it's not part of the library, nothing is to update here.

p.s. Don't forget to update uploadcare-php-example if necessary.

[vipulnsward]

Added an example of current supported events in example App, they are already supported via the gem.