Can't install up: The requested URL returned error: 415 Unsupported Media Type

## Prerequisites

* [x] I am running the latest version. (`up upgrade`)
* [x] I searched to see if the issue already exists.
* [x] I inspected the verbose debug output with the `-v, --verbose` flag.
* [ ] Are you an Up Pro subscriber?

## Description

When I install up, I get the following error: _The requested URL returned error: 415 Unsupported Media Type_

## Steps to Reproduce

Installation steps on your README.md

`curl -sfL https://raw.githubusercontent.com/apex/up/master/install.sh | sh`

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