releases#uploadAsset is not compatible with GitHub Enterprise

[kyleolsondesign]

The config passed when instantiating can set the host and pathPrefix properties, which allows us to use GitHub Enterprise servers for most requests, but the releases#uploadAsset route information hardcodes another github.com reference:
https://github.com/mikedeboer/node-github/blob/1c9571cbe4bdfd924f1231ee95992f32d5b8f62d/api/v3.0.0/routes.json#L3667

This makes this API inaccessible for GitHub Enterprise instances

[kyleolsondesign]

It looks like there are two differences between the GitHub public API and GitHub Enterprise (in API v3 at least):

1. In public api, host changes to uploads.github.com. In enterprise, the host remains the same
2. In enterprise, the pathPrefix changes from: /api/v3 to /api/uploads

[kyleolsondesign]

👍 Tested today. Works great!