Pin webrick to compatible version

Fixes elastic#8845

Fixes elastic#8846

Author: jakelandis

Gemfile.template

@@ -22,6 +22,7 @@ gem "flores", "~> 0.0.6", :group => :development
 gem "term-ansicolor", "~> 1.3.2", :group => :development
 gem "json-schema", "~> 2.6", :group => :development
 gem "pleaserun", "~>0.0.28"
+gem 'webrick', '~> 1.3.1'
 gem "logstash-input-heartbeat"
 gem "logstash-codec-collectd"
 gem "logstash-codec-cef"