First of all, rubygemsrecipe tries to use latest rubygems library when
rubygems library version is not explicitly specified. So according to
slapos approach, that recipes are immutable, we should pin it.

For fluentd, Rafael already pinned rubygems library version to 2.4.8
(see b4fa5d31 "Pin fluentd version.").

However rubygemsrecipe tries to both find latest rubygems library
version, and to download rubygems when only version was specified from

    http://production.cf.rubygems.org/rubygems/

https://bitbucket.org/sirex/rubygemsrecipe/src/a37d4859a342b9250aeca8977ed565514a00fee4/rubygems.py?at=default#rubygems.py-126
https://bitbucket.org/sirex/rubygemsrecipe/src/a37d4859a342b9250aeca8977ed565514a00fee4/rubygems.py?at=default#rubygems.py-119

That site however no longer works and according to

    https://rubygems.org/pages/download

rubygems library tarballs are directly downloadable from

    https://rubygems.org/rubygems/...

So to make fluentd SR let's change pinning to work by full URL.

Also pin all other places which use rubygemsrecipe (only
software/unstable/gitrepo/software.cfg for now).

/trusted-by @rafael