Use with_unbundled_env for Bundler 2.1+ (#2151)

lib/puma/launcher.rb

@@ -299,7 +299,7 @@ def prune_bundler
       log '* Pruning Bundler environment'
       home = ENV['GEM_HOME']
       bundle_gemfile = ENV['BUNDLE_GEMFILE']
-      Bundler.with_clean_env do
+      with_unbundled_env do
         ENV['GEM_HOME'] = home
         ENV['BUNDLE_GEMFILE'] = bundle_gemfile
         ENV['PUMA_BUNDLER_PRUNED'] = '1'
@@ -473,5 +473,14 @@ def require_rubygems_min_version!(min_version, feature)
       raise "#{feature} is not supported on your version of RubyGems. " \
               "You must have RubyGems #{min_version}+ to use this feature."
     end
+
+    def with_unbundled_env
+      bundler_ver = Gem::Version.new(Bundler::VERSION)
+      if bundler_ver < Gem::Version.new('2.1.0')
+        Bundler.with_clean_env { yield }
+      else
+        Bundler.with_unbundled_env { yield }
+      end
+    end
   end
 end