Lock UnCSS to v0.8.1 due to stability issues

In line with changes made in grunt-uncss (https://github.com/addyosmani/grunt-uncss/pull/111), it might be a good idea to lock UnCSS to version 0.8.1 as there seem to be stability issues with later versions.

Log with example of issue encountered: https://travis-ci.org/javiercejudo/javiercejudo.com/jobs/29311660 (see line 1198)