Add .byebug_history to gitignore

.byebug_history included in default Rails5 setup (https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/app/templates/gitignore)

Rails.gitignore

@@ -33,3 +33,6 @@ bower.json
 
 # Ignore pow environment settings
 .powenv
+
+# Ignore Byebug command history file.
+.byebug_history