18245: Update the "Ruby and Bundler" installation page:

18245: Update the "Ruby and Bundler" installation page:
- installing the system bundler package is no longer needed
- the RVM installation method already installs bundler, no need to do
  that explicitly
- update the references to the latest Ruby 2.5.x version
- remove reference to Ubuntu 16.04 which is no longer supported
- update the "install from source" documentation to make it work with
  the Arvados rails packages

Arvados-DCO-1.1-Signed-off-by: Ward Vandewege <ward@curii.com>