Replace with_instrumenter if not available

`with_connection` was removed in
rails/rails@dc522a3

This keeps things working for now, but we should consider finding
a better way to handle this, rather than reaching into AR's internals.

app/models/solid_cache/record.rb

@@ -10,7 +10,20 @@ class Record < ActiveRecord::Base
 
     class << self
       def disable_instrumentation(&block)
-        connection.with_instrumenter(NULL_INSTRUMENTER, &block)
+        with_instrumenter(NULL_INSTRUMENTER, &block)
+      end
+
+      def with_instrumenter(instrumenter, &block)
+        if connection.respond_to?(:with_instrumenter)
+          connection.with_instrumenter(instrumenter, &block)
+        else
+          begin
+            old_instrumenter, ActiveSupport::IsolatedExecutionState[:active_record_instrumenter] = ActiveSupport::IsolatedExecutionState[:active_record_instrumenter], instrumenter
+            block.call
+          ensure
+            ActiveSupport::IsolatedExecutionState[:active_record_instrumenter] = old_instrumenter
+          end
+        end
       end
 
       def with_shard(shard, &block)