Change to new challenge URLs

[robbertkl]

It seems a couple of days ago the challenge URLs were changed. This PR addresses this (they simply got renamed from /challenge to /challenges.

It works for me when using the latest release (2.0.0-alpha.12) of magister.js and then manually changing challenge to challenges. I wanted to submit a PR there with the rename, but noticed that since the last release, the authentication was moved to (replaced by) this library, so now I created my PR here.

When trying to get everything working with magister.js master branch and this library I ran into some issues:

• The auth code was not passed along and magister-openid introduced a breaking change in 0.1.4; I've solved this with this PR: Use authCode from config again simplyGits/MagisterJS#150
• The challenge URLs were not correct (that's THIS PR)
• Finally, it failed with TypeError: client_secret_basic client authentication method requires a client_secret in getTokenSet()

I haven't been able yet to solve this last error, but at least this PR gets it a few steps further down the authentication flow. I'll put the error in a separate issue. Please note I did not have this error when changing the challenge URL manually in an older version of magister.js.