Upgrade imagemin to mitigate deprecation warning

[mahnunchik]

npm WARN deprecated resolve-url@0.2.1: https://github.com/lydell/resolve-url#deprecated

npm list resolve-url
└─┬ hexo-asset-pipeline@1.8.0
  └─┬ imagemin@6.1.0
    └─┬ globby@8.0.2
      └─┬ fast-glob@2.2.7
        └─┬ micromatch@3.1.10
          └─┬ snapdragon@0.8.2
            └─┬ source-map-resolve@0.5.3
              └── resolve-url@0.2.1 

Imagemin has new version https://www.npmjs.com/package/imagemin

[mahnunchik]

The same:

npm WARN deprecated urix@0.1.0: Please see https://github.com/lydell/urix#deprecated

npm list urix       
└─┬ hexo-asset-pipeline@1.8.0
  └─┬ imagemin@6.1.0
    └─┬ globby@8.0.2
      └─┬ fast-glob@2.2.7
        └─┬ micromatch@3.1.10
          └─┬ snapdragon@0.8.2
            └─┬ source-map-resolve@0.5.3
              └── urix@0.1.0