point to new py-multiaddr location on readthedocs, remove old travis.yml

[pacrob]

I set up a new readthedocs location under py-multiaddr, this updates the docs link in the README to point there instead. Removed travis.yml as it is no longer used.

[pacrob]

Contacted @sbuss via x hoping to use the original multiaddr readthedocs project rather than creating a new py-multiaddr.

[sbuss]

Please see #87 for follow up