Depends on an older version of `mini_magick`

I am using a newer version of mini_magick. I get this error:

```
Bundler could not find compatible versions for gem "mini_magick":
  In snapshot (Gemfile.lock):
    mini_magick (= 4.6.1)

  In Gemfile:
    mini_magick (>= 4.6.1, ~> 4.6)

    webshot (~> 0.1.0) was resolved to 0.1.0, which depends on
      mini_magick (~> 4.3.3)
```