Adds github: to packages versions from github, breaking build

[Pezmc]

With dependencies declared as the following:

  "dependencies": {
    "ember-cli-bugsnag": "Pezmc/ember-cli-bugsnag#v2.0.0",
    "next-update": "^3.6.0"
  },

If you run next-update, it changes the package.json file to read:

  "dependencies": {
    "ember-cli-bugsnag": "github:Pezmc/ember-cli-bugsnag#v2.0.0",
    "next-update": "^3.6.0"
  },

Which breaks the build as there's no such version as github:Pezmc/ember-cli-bugsnag#v2.0.0.

I've put together a test repo to demonstrate this here: https://github.com/Pezmc/next-update-error