Update grunt-contrib-htmlmin requirement from ~2.4.0 to ~3.1.0

Updates the requirements on [grunt-contrib-htmlmin](https://github.com/gruntjs/grunt-contrib-htmlmin) to permit the latest version.
- [Release notes](https://github.com/gruntjs/grunt-contrib-htmlmin/releases)
- [Changelog](https://github.com/gruntjs/grunt-contrib-htmlmin/blob/master/CHANGELOG)
- [Commits](gruntjs/grunt-contrib-htmlmin@v2.4.0...v3.1.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

package.json

@@ -51,7 +51,7 @@
     "grunt-contrib-connect": "^0.10.1",
     "grunt-contrib-copy": "~0.5.0",
     "grunt-contrib-cssmin": "~0.9.0",
-    "grunt-contrib-htmlmin": "~0.3.0",
+    "grunt-contrib-htmlmin": "~3.1.0",
     "grunt-contrib-jshint": "~0.10.0",
     "grunt-contrib-uglify": "~0.4.0",
     "grunt-contrib-watch": "~0.6.1",