pins component/fluentd gems version to fix :

ERROR:  Error installing fluentd:
msgpack requires Ruby version >= 2.4.

encountered while supplying software/fluentd in a SlapOS Webrunner.

removes outdated gem arguments --no-ri --no-rdoc

by default rubygemsrecipe uses --no-document (see rubygems.py)

splits gem-options arguments to fit the README example (see README.rst)

adds minimal test to check fluentd SR instanciation