Upgrade to 2.1.9 and improve

[UnitedMarsupials-zz]

The patch I have improves things as follows:

• Upgrade to 2.1.9 -- supposedly, the last 2.1.x release ever.
• Switches from .tar.gz to .tar.xz -- the tarball is about 1/3 smaller.
• Separates /usr/share/doc and /usr/share/ri into a sub-package of its own (ruby-doc-2.1.9) -- people, who need, documentation can install it, but it is not needed on most servers.
• Removes tcl-devel from the build-requirement -- as long a Tk is explicitly disabled, there is no need for Tcl either.
• Runs bundled self-tests as %check.
• Uses simply %{version} instead of %{rubyver}.
• Adds rybygem-json to both Provides and Obsoletes -- whatever other RPMs require the gem, will now be satisfied.