Encountering error: uninitialized constant Fluent::Plugin::GrokParser::NoneParser (NameError)

Hello,

Fluentd version: 1.13.3
Grok parser plugin version: 2.0.1

I'm currently experiencing an issue when using the grok parser, where I receive an error during Fluentd startup:

```log
fluentd[24300]: /var/lib/gems/2.5.0/gems/fluent-plugin-grok-parser-2.0.1/lib/fluent/plugin/parser_grok.rb:17:in `initialize': uninitialized constant Fluent::Plugin::GrokParser::NoneParser (NameError)
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/plugin.rb:163:in `new'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/plugin.rb:163:in `new_impl'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/plugin.rb:137:in `new_parser'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/plugin_helper/parser.rb:89:in `block in configure'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/plugin_helper/parser.rb:85:in `each'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/plugin_helper/parser.rb:85:in `configure'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/plugin/in_tail.rb:134:in `configure'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/plugin.rb:178:in `configure'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/root_agent.rb:317:in `add_source'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/root_agent.rb:158:in `block in configure'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/root_agent.rb:152:in `each'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/root_agent.rb:152:in `configure'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/engine.rb:105:in `configure'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/engine.rb:80:in `run_configure'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/supervisor.rb:663:in `run_supervisor'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/lib/fluent/command/fluentd.rb:344:in `'
fluentd[24300]: from /usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
fluentd[24300]: from /usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'
fluentd[24300]: from /var/lib/gems/2.5.0/gems/fluentd-1.13.3/bin/fluentd:15:in `'
fluentd[24300]: from /usr/local/bin/fluentd:23:in `load'
fluentd[24300]: from /usr/local/bin/fluentd:23:in `'
```

My config section is:
```xml

@type tail
@id source_tail_openvpn
path /var/log/openvpn.log
pos_file /var/log/fluentd/pos_openvpn.log.pos
tag openvpn

@type grok
grok_pattern %{TIMESTAMP_ISO8601:@timestamp} %{GREEDYDATA:message}

```