Ruby warns "instance variable `@version` not initialized" for optparse when it receives `--version` option.

`test.rb`

```ruby
require 'optparse'
Version = '1'
OptionParser.new.parse(ARGV)
```

```
$ ruby -w test.rb --version
/home/pocke/.rbenv/versions/2.5.1/lib/ruby/2.5.0/optparse.rb:1168: warning: instance variable @version not initialized
/home/pocke/.rbenv/versions/2.5.1/lib/ruby/2.5.0/optparse.rb:1175: warning: instance variable @release not initialized
test 1
```

This change will suppress the warnings.

[Fix GH-1871]

From: Masataka Pocke Kuwabara <kuwabara@pocke.me>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63342 b2dd03c8-39d4-4d8f-98ff-823fe69b080e