verify Rubygems versions before monkeypatching

Fixes #2684

lib/logstash/environment.rb

@@ -11,27 +11,31 @@ require 'logstash/version'
 # see https://github.com/elasticsearch/logstash/issues/2556 and https://github.com/rubygems/rubygems/issues/1070
 #
 # this code is from Rubygems v2.1.9 in JRuby 1.7.17. Per tickets this issue should be solved at JRuby >= 1.7.20.
-class Gem::Specification
+
-  def self.reset
+# this method implementation works for Rubygems version 2.1.0 and up, verified up to 2.4.6
-    @@dirs = nil
+if Gem::Version.new(Gem::VERSION) >= Gem::Version.new("2.1.0") && Gem::Version.new(Gem::VERSION) < Gem::Version.new("2.5.0")
-    Gem.pre_reset_hooks.each { |hook| hook.call }
+  class Gem::Specification
-    @@all = nil
+    def self.reset
-    @@stubs = nil
+      @@dirs = nil
-    _clear_load_cache
+      Gem.pre_reset_hooks.each { |hook| hook.call }
-    unresolved = unresolved_deps
+      @@all = nil
-    unless unresolved.empty?
+      @@stubs = nil
-      unless (unresolved.size == 1 && unresolved["ffi"])
+      _clear_load_cache
-        w = "W" + "ARN"
+      unresolved = unresolved_deps
-        warn "#{w}: Unresolved specs during Gem::Specification.reset:"
+      unless unresolved.empty?
-        unresolved.values.each do |dep|
+        unless (unresolved.size == 1 && unresolved["ffi"])
-          warn "      #{dep}"
+          w = "W" + "ARN"
+          warn "#{w}: Unresolved specs during Gem::Specification.reset:"
+          unresolved.values.each do |dep|
+            warn "      #{dep}"
+          end
+          warn "#{w}: Clearing out unresolved specs."
+          warn "Please report a bug if this causes problems."
         end
-        warn "#{w}: Clearing out unresolved specs."
+        unresolved.clear
-        warn "Please report a bug if this causes problems."
       end
-      unresolved.clear
+      Gem.post_reset_hooks.each { |hook| hook.call }
     end
-    Gem.post_reset_hooks.each { |hook| hook.call }
   end
 end