add Jruby support

```ruby
img = MiniMagick::Image.open('http://jprice.360buyimg.com/price/gp321326-1-1-3.png')
img.colorspace("GRAY")
img.monochrome

str = RTesseract.new(img.path, processor: 'mini_magick').to_s.strip
File.unlink(img.path) # rm tmp file
price = str.match(/\d+\.\d+/)[0].to_f
puts "Price：#{price}"
```
Return
`=> 59.0`