Convert readthedocs links for their .org -> .io migration for hosted projects

As per [their blog post of the 27th April](https://blog.readthedocs.com/securing-subdomains/) ‘Securing subdomains’:

> Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.

Test Plan: Manually visited all the links I’ve modified.

README.rst

@@ -16,7 +16,7 @@ indentation, etc.
    :target: https://coveralls.io/github/adrienverge/yamllint?branch=master
    :alt: Code coverage status
 .. image:: https://readthedocs.org/projects/yamllint/badge/?version=latest
-   :target: http://yamllint.readthedocs.org/en/latest/?badge=latest
+   :target: https://yamllint.readthedocs.io/en/latest/?badge=latest
    :alt: Documentation status
 
 Written in Python (compatible with Python 2 & 3).
@@ -24,7 +24,7 @@ Written in Python (compatible with Python 2 & 3).
 Documentation
 -------------
 
-http://yamllint.readthedocs.io/
+https://yamllint.readthedocs.io/
 
 Overview
 --------
@@ -82,7 +82,7 @@ Usage
  # Output a parsable format (for syntax checking in editors like Vim, emacs...)
  yamllint -f parsable file.yaml
 
-`Read more in the complete documentation! <http://yamllint.readthedocs.io/>`_
+`Read more in the complete documentation! <https://yamllint.readthedocs.io/>`_
 
 Features
 ^^^^^^^^
@@ -119,7 +119,7 @@ or for a whole block:
    consectetur : adipiscing elit
  # yamllint enable
 
-`Read more in the complete documentation! <http://yamllint.readthedocs.io/>`_
+`Read more in the complete documentation! <https://yamllint.readthedocs.io/>`_
 
 License
 -------