Update the link to the cashaddr spec

Summary: The link to the cashaddr spec on https://www.bitcoinabc.org/cashaddr/ and https://www.bitcoinabc.org/2018-01-14-CashAddr-es/ points to the deprecated repository https://github.com/bitcoincashorg/spec/blob/master/cashaddr.md.

Test Plan: none (Failed to install rvm). If necessary, I can test tomorrow morning on another OS.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7277