Explicitly mark command methods private

The `execute` method is the only public method exposed by commands at
this time.

lib/rom/csv/commands/create.rb

@@ -17,6 +17,8 @@ def execute(tuples)
           insert_tuples
         end
 
+        private
+
         def insert(tuples)
           tuples.each { |tuple| dataset << new_row(tuple) }
           dataset.sync!

lib/rom/csv/commands/delete.rb

@@ -19,6 +19,8 @@ def execute
           dataset.data
         end
 
+        private
+
         def dataset
           relation.dataset
         end

lib/rom/csv/commands/update.rb

@@ -14,6 +14,8 @@ def execute(tuple)
           update(tuple)
         end
 
+        private
+
         def update(tuple)
           original_data = original_dataset.to_a
           output = []