component/fluentd: fix failing install pinning gems version

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• removes outdated gem arguments --no-ri --no-rdoc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