Fix filebeat::prospector::fields_under_root

[KrzysztofHajdamowicz]

filebeat::prospector::fields_under_root had hardcoded value instead of following global variable set for whole module filebeat, thus setting global filebeat::fields_under_root: true had no effect.

[KrzysztofHajdamowicz]

Only check that failed fails at:

$ gem update --system
Updating rubygems-update
Fetching: rubygems-update-3.0.2.gem (100%)
ERROR:  Error installing rubygems-update:
	There are no versions of rubygems-update (= 3.0.2) compatible with your Ruby & RubyGems
	rubygems-update requires Ruby version >= 2.3.0. The current ruby version is 2.1.0.
ERROR:  While executing gem ... (Errno::ENOENT)
    No such file or directory @ dir_chdir - /home/travis/.rvm/gems/ruby-2.1.9/gems/rubygems-update-2.7.8
The command "gem update --system" failed and exited with 1 during .

IMO this affect whole puppet module than my patch.