refactor: Remove stroke_opacity before overriding to get rid of warning message (#373)
refactor: Remove stroke_opacity before overriding to get rid of warning message (#373)

This patch will remove following warning message.

```
$ bundle exec rake
/opt/gruff/lib/gruff/patch/rmagick.rb:16: warning: method redefined; discarding old stroke_opacity
```
topfunky/gruff,topfunky/gruff