Update the list under Music Distribution, remove songriffer

[unicast]

• Spinup is closed (as per their website https://spinnup.com/)
• Soundcloud - updated the link to their music distribution landing page
• Added Amuse as one of the growing music distributors

Also songriffer seems pivoted towards digital money transfers so I removed it from the list.

[unicast]

@ad-si it would be great if you can review and merge my PR. I also see there are more PRs are open for some time already.