replace `JSON` `dump`/`load` with `parse`/`generate`

`dump` and `load` are for built around Marshaling ruby objects generally.
They correspond with those methods on Ruby's `Marshal` class. Theses
methods actually call `parse`/`generate` in code but pass some defaults
along with it. Sprockets only needs to parse JSON documents not Ruby
objects.

For `JSON.load`, we want to disable `create_additions`.
`create_additions` could be considered a security hazard if set to true.
`create_additions` allows the instantiation of any class that's
marshaled as json