$ref in generated files are mixed case, but generated files are lower case

The files generated by the tool are all lowercase file names because of this `.lower()` (and the subsequent ones?):

https://github.com/instrumenta/openapi2jsonschema/blob/d697cbff8a25f520e125e3a5f79cb4e9b972e8ce/openapi2jsonschema/command.py#L130

But the `$ref`s are the case of the input, in my case mixed case:

https://github.com/instrumenta/openapi2jsonschema/blob/d697cbff8a25f520e125e3a5f79cb4e9b972e8ce/openapi2jsonschema/command.py#L222

The result is a schema that doesn't load properly.

_This tool pretty much did exactly what I needed, thanks!_

As a workaround, I lowercased the `$ref` lines in all the json files.