Add AppVeyor to run tests against Windows CI

[ci skip]
Don't define MirrorCommand if already defined.
Fix typo in platform= documentation
Merge pull request #1048 from jasonrclark/fix_platform_typo

Fix typo in platform= documentation [ci skip]
Add #1048 to History start RG version 2.5 section
Update travis to latest 2.1 release
Merge pull request #1044 from akr/fix-mirror

Don't define MirrorCommand if already defined.
Add #1044 to History
backport r48268 from ruby/ruby trunk
Add CA roots for upcoming rubygems.org cert change

We need to deploy a new server certificate as Google Chrome no longer
trusting SHA-1-signed certificates:

  http://blog.chromium.org/2014/09/gradually-sunsetting-sha-1.html

In order to avoid interruption of service users will need the updated
root certificates to verify the new server certificate.  Users may be
able to install these certificates by hand into their OpenSSL
certificates pem as well.
Update release date and version
Update Manifest
backport r48370 from ruby/ruby trunk
backport r48371 from ruby/ruby trunk
Remove COMODO intermediate certificates

Root certificates are the ones that matters in this scenario.
Add Root CA with SHA1 signature

This is required for some versions of OpenSSL having trouble
to establish and verify the new certificates for rubygems.org

SHA-1-based signatures for trusted root certificates are not a
problem because TLS clients trust them by their identity, rather
than by the signature of their hash.

http://blog.chromium.org/2014/09/gradually-sunsetting-sha-1.html
Update Manifest.txt

Reflect latest changes in certs.
Update History.txt with recent changes

Missed include details in past commits. I'm getting rusty at OSS...
Add staging to CA certificate checker

Also show which certificate each URL needs

Also warn when overwriting an existing certificate (even though git
knows).
Set release date and version