blacklist grunt-favicons-tasks

With this plugin you can't do anything more/easier than using the [`favicons` module](https://github.com/creunabo/favicons) directly.

[Also, this plugin seems to be grunt focussed](https://github.com/creunabo/favicons/commit/7152f6d328239bbfa4d4da923e18aee6b15a7d3a).

Cc: @creunabo